Canonicalize variable usage in testsuite Makefiles

This test streamlines our use of variables that are expected by
Makefile.rules throughout the test suite. Mostly it replaced
potentially dangerous overrides and updates of variables like CFLAGS
with safe assignments to variables reserved for this purpose like
CFLAGS_EXTRAS.

Differential Revision: https://reviews.llvm.org/D67984

llvm-svn: 372795