Install plain.test.mk.

This was missed when this file was first imported.  Its atf.test.mk
counterpart is already being installed and these are necessary if we
want "make" within the source tree (not via "buildworld") to work.

Approved by:	rpaulo (mentor)
Generate plain sh test programs from a source file.

Instead of assuming that plain sh test programs exist in the source
tree in their final form and are marked as executable, generate them
from a list of sources.

By default, just assume that the source file for a program P is P.sh
but allow the caller to customize the inputs.  Similarly, also allow
the caller to apply sed(1) replacements on the output.  These will
both be useful in hooking existing test code from tools/regression/
into the test suite.

Approved by:	rpaulo (mentor)