Combine more conformtest tests into single execution of the compiler.

In <https://sourceware.org/ml/libc-alpha/2018-11/msg00225.html>,
Florian reported that the change from conformtest.pl to conformtest.py
had increased conform/ test time, possibly because of increased
startup overhead for Python scripts.

This patch improves conformtest.py performance by arranging for as
many tests of a (header, standard) pair as possible to use a single
execution of the compiler, so it does not need to initialize and parse
the whole header under test separately for every test assertion.
Specifically, compilation tests that are not marked as "optional" or
"xfail" are combined into a single source file, and are only then run
separately if compilation of that combined file fails.  For me, this
reduces the wall clock time for the conformtest.py tests (not the
whole of the conform/ directory) from two minutes to 15 seconds.

Tested for x86_64, and with build-many-glibcs.py.

	* conform/conformtest.py (CompileSubTest.__init__): Set
	self.run_early to False.
	(ExecuteSubTest.__init__): Likewise.
	(HeaderTests.run): Try running all non-optional, non-XFAILed
	compilation tests in a single execution of the compiler.