testsuite: outputs.exp: test for skip_atsave more thoroughly

testsuite: outputs.exp: test for skip_atsave more thoroughly

The presence of -I or -L flags in link command lines changes the
driver's, and thus the linker's behavior, WRT naming files with
command-line options.  With such flags, the driver creates .args.0 and
.args.1 files, whereas without them it's the linker (collect2, really)
that creates .ld1_args.

I've hit some fails on a target system that doesn't have -I or -L
flags in the board config file, but it does add some of them
implicitly with configured-in driver self specs.  Alas, the test in
outputs.exp doesn't catch that, so we proceed to run rather than
skip_atsave tests.

I've reworked the outest procedure to allow dry runs and to return
would-have-been pass/fail results as lists, so we can now test whether
certain files are created and use that to configure the actual test
runs.

for  gcc/testsuite/ChangeLog

	* gcc.misc-tests/outputs.exp (outest): Introduce quiet mode,
	create and return lists of passes and fails.  Use it to catch
	skip_atsave cases where -L flags are implicitly added by
	driver self specs.