test: Non-existing component won't match git SHA

The #component_matches_git_sha? runs a git command to see if the
component is at the correct version. To run it, it uses
Gitlab::Popen, but as a side-effect it creates the directory before it
runs the command. This causes the rake task gitlab:gitaly:clone to
incorrectly determine if a git-clone(1) is needed.

To speed things up, and avoid creation of an empty directory, this
change adds an additional check to see if the component's folder exists
anyway.

spec/support/helpers/test_env.rb

@@ -594,6 +594,8 @@ module TestEnv
     # Not a git SHA, so return early
     return false unless expected_version =~ ::Gitlab::Git::COMMIT_ID
 
+    return false unless Dir.exist?(component_folder)
+
     sha, exit_status = Gitlab::Popen.popen(%W(#{Gitlab.config.git.bin_path} rev-parse HEAD), component_folder)
     return false if exit_status != 0