tests: use $GREP throughout.

tests: use $GREP throughout.

Trust the user, and use $GREP from TESTS_EVVIRONMENT uniformly.
* Makefile.am (TESTS_ENVIRONMENT): Add GREP setting.
* tests/am-subdir.at, tests/archive-in-archive.at, tests/bindir.at,
tests/configure-iface.at, tests/cwrapper.at, tests/darwin.at,
tests/demo.at, tests/destdir.at, tests/dumpbin-symbols.at,
tests/early-libtool.at, tests/execute-mode.at, tests/export-def.at,
tests/export.at, tests/f77demo.at, tests/fcdemo.at, tests/flags.at,
tests/help.at, tests/inherited_flags.at, tests/install.at,
tests/lalib-syntax.at, tests/libtool.at, tests/libtoolize.at,
tests/localization.at, tests/lt_dladvise.at, tests/lt_dlopen.at,
tests/lt_dlopenext.at, tests/ltdl-api.at, tests/ltdl-libdir.at,
tests/mdemo.at, tests/old-m4-iface.at, tests/pic_flag.at,
tests/resident.at, tests/runpath-in-lalib.at, tests/search-path.at,
tests/static.at, tests/sysroot.at, tests/template.at,
tests/testsuite.at, tests/versioning.at: Replace all hardcoded grep
invocations with $GREP.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>