Fix atf-sh's integration_test

With the move of atf-sh into /usr/libexec in r267181, some of the
tests in the integration_test program broke because they could not
execute atf-sh from the path any longer.

This slipped through because I do have a local atf installation in
my home directory that appears in my path, hence the tests could
still execute my own version.

Fix this by forcing /usr/libexec to appear at the beginning of the
path when attempting to execute atf-sh.

To make upgrading easy (and to avoid an unnecessary entry in UPDATING),
make integration_test depend on the Makefile so that a rebuild of the
shell script is triggered.  This requires a hack in the *.test.mk files
to ensure the Makefile is not treated as a source to the generated
program.  Ugly, I know, but I don't have a better way of doing this at
the moment.  Will think of one once I address the TODO in the *.test.mk
files that suggests generalizing the file generation functionality.

PR:		191052
Reviewed by:	Garrett Cooper