patchtest: test_metadata: fix invalid escape sequences

Clear up the following warnings seen during patchtest runs:

|/workspace/yocto/poky/meta/lib/patchtest/tests/test_metadata.py:21: SyntaxWarning: invalid escape sequence '\+'
|  add_mark = pyparsing.Regex('\+ ')
|/workspace/yocto/poky/meta/lib/patchtest/tests/test_metadata.py:26: SyntaxWarning: invalid escape sequence '\:'
|  git_regex = pyparsing.Regex('^git\:\/\/.*')

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>