patchtest: don't match BitBake python expansion as GitHub username

patchtest: don't match BitBake python expansion as GitHub username

BitBake's Python expansion syntax looks very similar to GitHub usernames:
${@foo} - which triggers a false alarm from patchtest.

This patch adds a negative lookahead to the GitHub username matching pattern:
only match in case the pattern doesn't start with "${" characters.

Also add a test for it.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>