Polish libstdc++ 'dg-final' action 'file-io-diff'

Polish libstdc++ 'dg-final' action 'file-io-diff'

Follow-up to recent commit 515da03a838db05443ebcc4c543a405bed764188
"libstdc++: Add file-io-diff to replace @diff@ markup in I/O tests".

Currently, if a 'dg-final' action 'file-io-diff' passes, we print nothing
(should: 'PASS: [...]'), but if it fails, we just print: 'FAIL: files differ',
for example ('*.log' file):

    [...]
    FAIL: 27_io/basic_ostream/inserters_other/wchar_t/2.cc  -std=gnu++17 (test for excess errors)
    [...]
    UNRESOLVED: 27_io/basic_ostream/inserters_other/wchar_t/2.cc  -std=gnu++17 compilation failed to produce executable
    diff: wostream_inserter_other_in.txt: No such file or directory
    diff: wostream_inserter_other_out.txt: No such file or directory
    FAIL: files differ
    diff: wostream_inserter_other_in.txt: No such file or directory
    diff: wostream_inserter_other_out.txt: No such file or directory

When later the '*.sum' files get sorted, these 'FAIL: files differ' instances
aren't grouped anymore with the other test cases' results, but they appear en
bloc, lexically sorted between ('e[...]' and 's[...]'), for example:

    [...]
    PASS: ext/vstring/types/23767.cc  -std=gnu++17 (test for excess errors)
    FAIL: files differ
    FAIL: files differ
    FAIL: files differ
    PASS: special_functions/01_assoc_laguerre/check_nan.cc  -std=gnu++17 (test for excess errors)
    [...]

Also, we shouldn't emit the actual 'diff' into the '*.sum' file, but just into
the '*.log* file, and there's no need for 'spawn'/'expect', as we're not
matching any specific messages.

	libstdc++-v3/
	* testsuite/lib/libstdc++.exp (file-io-diff): Polish.