gdb/testsuite: avoid creating temp file in gdb/testsuite/ directory

gdb/testsuite: avoid creating temp file in gdb/testsuite/ directory

After this commit:

  commit 33c1395cf5e9deec7733691ba32c450e5c27f757
  Date:   Fri Nov 11 15:26:46 2022 +0000

      gdb/testsuite: fix gdb.trace/unavailable-dwarf-piece.exp with Clang

The gdb.trace/unavailable-dwarf-piece.exp test script was creating a
temporary file in the build/gdb/testsuite/ directory, instead of in
the expected place in the outputs directory.

Fix this by adding a call to standard_output_file.