Add unit-tests for .MAKE.META.IGNORE_*

These unit-tests should only be run if TEST_MAKE
has a valid .MAKE.PATH_FILEMON to indicate that filemon is
supported.

All three tests use the same meta-ignore.inc
which runs three sub-makes

one to initialize the target .meta file
two to ignore a file using .MAKE.META.IGNORE_{PATHS,PATTERNS,FILTER}
three to verify that without .MAKE.META.IGNORE_* the target is out of date.

Fix the order of _SED_CMDS to avoid errors when ${MAKE:T} appears in
.OBJDIR

make(1): describe the purpose of each newly added unit test dummy

make(1): add dummies for fine-grained tests, one per single feature

The test names have been derived from the current manual page.

All these tests are dummies right now, and the code from the existing
tests will be moved into the new tests step by step.

This is done to prevent modmisc, escape, varmod-edge and varmisc from
growing without any bounds, and to reduce the side-effects of one test
to the others.