tests: Use separate files for strip outputs

This way we can verify that strip actually created all of them.
Also, check that strip, unstrip, elflint actually succeed in
run-strip-test-many.sh. This exposed the fact that the generated
testfile in run-strip-test-many.sh is considered as illformed by
strip -g.

Signed-off-by: Ulf Hermann <ulf.hermann@qt.io>