Do not generate UNRESOLVED results for run-built-tests = no.

Testing with run-built-tests = no generates many UNRESOLVED results in
tests.sum (and so in the output of "make check"), for all the tests
that are only compiled and not run in such a configuration.  This
doesn't seem useful in the "make check" output, and also causes "make
check" to exist with error status even when all tests that can be run
in such a configuration passed.

This patch changes it not to consider those tests when generating
subdir-tests.sum, and so tests.sum, so that you get a smaller number
of tests considered in the final results rather than a huge pile of
UNRESOLVED.

Tested with a cross-compiler to ARM in a run-built-tests = no
configuration.

	* Rules (tests-expected): New variable, depending on
	$(run-built-tests).
	(tests): Pass $(tests-expected) to merge-test-results.sh, not
	$(tests).