[gdb/testsuite] Use untested in gdb.debuginfod/fetch_src_and_symbols.exp

[gdb/testsuite] Use untested in gdb.debuginfod/fetch_src_and_symbols.exp

With running test-case gdb.debuginfod/fetch_src_and_symbols.exp with target
board unix/-bad, I get:
...
gcc: error: unrecognized command line option '-bad'^M
compiler exited with status 1
gdb compile failed, gcc: error: unrecognized command line option '-bad'
FAIL: gdb.debuginfod/fetch_src_and_symbols.exp: compile
...

Replace the FAIL with the usual:
...
UNTESTED: gdb.debuginfod/fetch_src_and_symbols.exp: failed to compile
...

Tested on x86_64-linux.