tests/e2e,etcdctl,etcdmain: Fix go test --tags cov -v ./tests/e2e

This CL fixes:
  COVERDIR=./coverage PASSES="build_cov" && go test --tags cov -v ./tests/e2e
and is part of the effort to make:
  COVERDIR=coverage PASSES="build_cov cov" ./test
fully pass.

The args passed to ./bin/etcd_test and ./bin/etcdctl_test binaries were
mismatched. The protocol of passing the arguments using
environment variables has been replaces with proper passing of flags.

How the measurement of coverage by e2e tests works:
  1. COVERDIR=./coverage PASSES="build_cov" are generating
./bin/etcd_test and ./bin/etcdctl_test binaries.

  2. These binaries are tests (as coverage can be computed only for
tests) [see ./main_test.go ./etcdctl/main_test.go], but this tests are
running the main logic of the server and uppon termination (or SIGTERM
signal) are writting proper .coverprofile files in the $COVERDIR folder.
The binaries used to take arguments using env variables, but its not
needed any longer. The binaries can consume any command line arguments
that either test (so --test.fooo) or the original binary can consume.

 3.  The tests/e2e (when compiled with the --tags cov) are starting the
_test binaries instead of the original binaries, such that the coverage
is being collected.

test

@@ -304,6 +304,9 @@ function cov_pass {
 	# strip out generated files (using GNU-style sed)
 	sed --in-place '/generated.go/d' "$COVERDIR"/cover.out || true
 
+	echo -e "\nTo generate coverage report use:"
+	echo -e "  go tool cover --html=$COVERDIR/cover.out\n"
+
 	# held failures to generate the full coverage file, now fail
 	if [ -n "$failed" ]; then
 		for f in $failed; do
@@ -665,6 +668,7 @@ function build_cov_pass {
 	out="bin"
 	if [ -n "${BINDIR}" ]; then out="${BINDIR}"; fi
 	go test -mod=mod -tags cov -c -covermode=set -coverpkg="$PKGS_COMMA" -o "${out}/etcd_test"
+
 	go test -mod=mod -tags cov -c -covermode=set -coverpkg="$PKGS_COMMA" -o "${out}/etcdctl_test" "${REPO_PATH}/etcdctl"
 }