Turn the 4 side setup of the firststrike_and_laststrike test into a macro
called `COMMON_KEEP_A_B_C_D`for reuse in other tests.
The comment for `GENERIC_UNIT_TEST` gets a couple of extra paragraphs.
All the files get wmlindent run on them, which reindents `FAIL_IF_NOT`.
The main reason for having these is separate files is Git's fuzzy-patch
matching when merging and rebasing. Having large blocks of identical text
in `wml_unit_test_macros.cfg` can mean the wrong section gets patched.
