patchtest: rework license checksum tests

patchtest: rework license checksum tests

Remove the pretest_lic_files_chksum_modified_not_mentioned test entirely
and use pyparsing in test_lic_files_chksum_modified_not_mentioned to
scan the patches for lines starting with either "+LIC_FILES_CHKSUM" or
"-LIC_FILES_CHKSUM".  If either is found but no "License-Update" tag is
present in the commit, fail the test.

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