tests: use $SED throughout.

tests: use $SED throughout.

Trust the user, and use $SED from TESTS_ENVIRONMENT uniformly.
Hardcoding 'sed' for "simple" edits is a premature optimisation.
* tests/cdemo.at, tests/cmdline_wrap.at, tests/cwrapper.at,
tests/darwin.at, tests/demo.at, tests/duplicate_conv.at,
tests/duplicate_members.at, tests/execute-mode.at, tests/export.at,
tests/inherited_flags.at, tests/install.at, tests/libtoolize.at,
tests/mdemo.at, tests/need_lib_prefix.at, tests/old-m4-iface.at,
tests/sysroot.at, tests/tagdemo.at, tests/testsuite.at: Replace all
hardcoded sed invocations with $SED.

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