Use temp-directory that is covered by framework level cleanup

Prior to this PR, the e2e tests where creating dirs like:
```
/tmp/testname1.etcd030299846
/tmp/testname0.etcd039445123
/tmp/testname0.etcd206372065
```
and not cleaning them, that led to disk-space-exceeded flakes.

After the PR, the testing.TB tempdir mechanism is used and the names are
being cleaned and are more miningful:

```
../../bin/etcd --name test-TestCtlV3EndpointHashKV-2 --listen-client-urls http://localhost:20010 --advertise-client-urls http://localhost:20010 --listen-peer-urls https://localhost:20011 --initial-advertise-peer-urls https://localhost:20011 --initial-cluster-token new --data-dir /tmp/TestCtlV3EndpointHashKV429176179/003 --snapshot-count 100000 --experimental-initial-corrupt-check --peer-auto-tls --initial-cluster test-TestCtlV3EndpointHashKV-0=https://localhost:20001,test-TestCtlV3EndpointHashKV-1=https://localhost:20006,test-TestCtlV3EndpointHashKV-2=https://localhost:20011
```

test.sh

@@ -67,7 +67,7 @@ fi
 
 # This options make sense for cases where SUT (System Under Test) is compiled by test.
 COMMON_TEST_FLAGS=("${RACE}")
-if [[ ! -z "${CPU}" ]]; then
+if [[ -n "${CPU}" ]]; then
   COMMON_TEST_FLAGS+=("--cpu=${CPU}")
 fi