Reformat code according to 'gotip' rules.

In practices adds annotations in the new syntax:
```
+//go:build !linux
 // +build !linux
```

Fixes failing gotip PASSES='fmt' check:
https://travis-ci.com/github/etcd-io/etcd/jobs/486453806

tests/e2e/cluster_direct_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cluster_proxy
 // +build !cluster_proxy
 
 package e2e

tests/e2e/cluster_proxy_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build cluster_proxy
 // +build cluster_proxy
 
 package e2e

tests/e2e/ctl_v3_auth_no_proxy_test.go

@@ -14,6 +14,7 @@
 
 // These tests depends on certificate-based authentication that is NOT supported
 // by gRPC proxy.
+//go:build !cluster_proxy
 // +build !cluster_proxy
 
 package e2e

tests/e2e/ctl_v3_watch_cov_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build cov
 // +build cov
 
 package e2e

tests/e2e/ctl_v3_watch_no_cov_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cov
 // +build !cov
 
 package e2e

tests/e2e/etcd_spawn_cov.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build cov
 // +build cov
 
 package e2e

tests/e2e/etcd_spawn_nocov.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cov
 // +build !cov
 
 package e2e

tests/e2e/v2_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !v2v3
 // +build !v2v3
 
 package e2e

tests/e2e/v2v3_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build v2v3
 // +build v2v3
 
 package e2e

tests/e2e/v3_cipher_suite_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cov && !cluster_proxy
 // +build !cov,!cluster_proxy
 
 package e2e

tests/integration/clientv3/connectivity/black_hole_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cluster_proxy
 // +build !cluster_proxy
 
 package connectivity_test

tests/integration/clientv3/connectivity/network_partition_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cluster_proxy
 // +build !cluster_proxy
 
 package connectivity_test

tests/integration/clientv3/watch_fragment_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cluster_proxy
 // +build !cluster_proxy
 
 package clientv3test

tests/integration/cluster_direct.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cluster_proxy
 // +build !cluster_proxy
 
 package integration

tests/integration/cluster_proxy.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build cluster_proxy
 // +build cluster_proxy
 
 package integration

tests/integration/embed/embed_proxy_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build cluster_proxy
 // +build cluster_proxy
 
 // The purpose of this (empty) package is too keep following test working:

tests/integration/embed/embed_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !cluster_proxy
 // +build !cluster_proxy
 
 // Keep the test in a separate package from other tests such that

tests/integration/v2store/store_tag_not_v2v3_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !v2v3
 // +build !v2v3
 
 package v2store_test

tests/integration/v2store/store_tag_v2v3_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build v2v3
 // +build v2v3
 
 package v2store_test