objdump -S test fail on mingw

objdump -S test fail on mingw

FAIL: objdump -S
FAIL: objdump --source-comment
is seen on mingw for the simple reason that gcc adds a .exe suffix on
the output file if not already present.  Fix that, and tidy some objcopy
tests.

	* testsuite/lib/binutils-common.exp (exeext): New proc.
	* testsuite/binutils-all/objcopy.exp (exe, test_prog): Use it here.
	(objcopy_remove_relocations_from_executable): Catch objcopy errors.
	Only run on ELF targets.
	* testsuite/binutils-all/objdump.exp (exe): Set variable.
	(test_build_id_debuglink, test_objdump_S): Use exe file suffix.