make(1): add enough tests to cover the ApplyModifier functions

Only a few return statements are still missing from the code coverage.

In ApplyModifier_Assign, the test for an empty variable name is skipped
for now since it segfaults.

In ApplyModifier_SysV after the second ParseModifierPart, the branch for
the missing delimiter is not reached since this case is already checked
for in the first part of the function. To trigger this branch, a
specially crafted, unrealistic string needs to be created, and that's too
complicated for the moment.

(rillig)