test: Ignore failures to remove the output file in some cases

When running tests on Windows with busybox-w32 as shell, removing
the output log file for the stacksmash-asan testcase fails like this
occasionally:

    rm: can't remove 'log1-stacksmash-asan': Permission denied

This only seems to happen with the i686 build of busybox-w32, but
happens for the stacksmash-asan test built as either i686 or x86_64.

(If running run-test.sh repeatedly, it seems like the issue either
happens right away on the first iteration, or doesn't happen at all.)

Avoid breaking builds by simply making it ignore errors from this
particular "rm" command.

test/Makefile

@@ -351,7 +351,7 @@ $(FAILURE_TEST_TARGETS): .failtested.%$(EXEEXT): %$(EXEEXT) .copied.%$(EXEEXT) $
 			exit 1; \
 			;; \
 		esac; \
-		rm -f $$OUT; \
+		rm -f $$OUT || true; \
 	fi
 	@touch $@