MFC r267176, r267181, r268445 (ATF-related commits):

Phabric: https://reviews.freebsd.org/D706
Approved by: rpaulo (mentor)
Approved by: re (gjb)
Reviewed by: jmmv
Sponsored by: EMC / Isilon Storage Division

r267176:

 Add the *_TESTS_SH_SED_* functionality to atf.test.mk.

 This exists already in plain.test.mk and tap.test.mk and should have been
 added to atf.test.mk too when the feature was first introduced.

 (It is probably time to address the related TODOs but I will do that
 separately.)

r267181:

 Move atf-sh from /usr/bin/ to /usr/libexec/

 In r266650, we made libatf-c and libatf-c++ private libraries so that no
 components outside of the source tree could unintendedly depend on them.

 This change does the same for the "atf-sh library" by moving the atf-sh
 interpreter from its public location in /usr/bin/ to the private location
 in /usr/libexec/.  Our build system will ensure that our own test programs
 use the right binary, but users won't be able to depend on atf-sh by
 "mistake".

 Committing this now to ride the UPDATING notice added with r267172 today.

r268445:

 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