pkg/testutil: Fixing flakes due to >>leak" text/template/parse goroutines.
pkg/testutil: Fixing flakes due to >>leak" text/template/parse goroutines.

Examplar flake: https://travis-ci.com/github/etcd-io/etcd/jobs/388806782
```
go test -timeout=5m -cpu=1 --run=Example ./client/...

ok  	go.etcd.io/etcd/v3/client	0.085s
testing: warning: no tests to run
PASS
Unexpected goroutines running after all test(s).
1 instances of:
text/template/parse.(*lexer).emit(...)
	/usr/local/go/src/text/template/parse/lex.go:157
text/template/parse.lexText(...)
	/usr/local/go/src/text/template/parse/lex.go:269 +0x4f0
text/template/parse.(*lexer).run(...)
	/usr/local/go/src/text/template/parse/lex.go:230 +0x37
created by text/template/parse.lex
	/usr/local/go/src/text/template/parse/lex.go:223 +0x190
FAIL	go.etcd.io/etcd/v3/client/integration	0.013s
```

ajityagaty/etcd,gyuho/etcd,li-ang/etcd,etcd-io/etcd,ajityagaty/etcd,coreos/etcd,mqliang/etcd,mkumatag/etcd,rnaveiras/etcd,mkumatag/etcd,coreos/etcd,mkumatag/etcd,etcd-io/etcd,etcd-io/etcd,mqliang/etcd,ajityagaty/etcd,rnaveiras/etcd,rnaveiras/etcd,gyuho/etcd,mqliang/etcd,li-ang/etcd