Attempt to make test in r367165 more robust.

Some people were seeing this failure:

```
: 'RUN: at line 83';   clang -mrelax-all -fno-integrated-as /b/s/w/ir/k/llvm-project/clang/test/Driver/as-options.s -S 2>&1 \
         | /FileCheck --check-prefix=WARN --allow-empty clang/test/Driver/as-options.s
--
Exit Code: 1

Command Output (stderr):
--
clang/test/Driver/as-options.s:66:16: error: NOWARN-NOT: excluded string found in input
// NOWARN-NOT: unused
               ^
<stdin>:1:95: note: found here
clang-10: warning: clang/test/Driver/as-options.s: 'assembler' input unused [-Wunused-command-line-argument]
```

Maybe this helps with that.

llvm-svn: 367176