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.)

share/mk/atf.test.mk

@@ -98,10 +98,14 @@ _TESTS+= ${ATF_TESTS_SH}
 SCRIPTSDIR_${_T}= ${TESTSDIR}
 TEST_INTERFACE.${_T}= atf
 CLEANFILES+= ${_T} ${_T}.tmp
+# TODO(jmmv): It seems to me that this SED and SRC functionality should
+# exist in bsd.prog.mk along the support for SCRIPTS.  Move it there if
+# this proves to be useful within the tests.
+ATF_TESTS_SH_SED_${_T}?= # empty
 ATF_TESTS_SH_SRC_${_T}?= ${_T}.sh
 ${_T}: ${ATF_TESTS_SH_SRC_${_T}}
 	echo '#! /usr/bin/atf-sh' > ${.TARGET}.tmp
-	cat ${.ALLSRC} >> ${.TARGET}.tmp
+	cat ${.ALLSRC} | sed ${ATF_TESTS_SH_SED_${_T}} >>${.TARGET}.tmp
 	chmod +x ${.TARGET}.tmp
 	mv ${.TARGET}.tmp ${.TARGET}
 .endfor