T17414 breaks if TMP is on a small tmpfs

T17414 tries to write a large (multi-gigabyte) file. However, this breaks if TMP (and therefore the test run directory) is on a small tmpfs:

Traceback (most recent call last):
  File "/mnt/work/ghc/ghc-compare-3/testsuite/driver/testlib.py", line 973, in test_common_work
    do_test(name, way, func, args, files)
  File "/mnt/work/ghc/ghc-compare-3/testsuite/driver/testlib.py", line 1071, in do_test
    result = func(*[name,way] + args)
  File "/mnt/work/ghc/ghc-compare-3/testsuite/driver/testlib.py", line 1352, in compile_and_run
    return compile_and_run__( name, way, None, [], extra_hc_opts)
  File "/mnt/work/ghc/ghc-compare-3/testsuite/driver/testlib.py", line 1349, in compile_and_run__
    return simple_run( name, way, cmd, getTestOpts().extra_run_opts )
  File "/mnt/work/ghc/ghc-compare-3/testsuite/driver/testlib.py", line 1591, in simple_run
    exit_code = runCmd(cmd, stdin_arg, stdout_arg, stderr_arg, opts.run_timeout_multiplier)
  File "/mnt/work/ghc/ghc-compare-3/testsuite/driver/testlib.py", line 2263, in runCmd
    stderr.write_bytes(stderr_buffer)
  File "/nix/store/4c4ajgdnhlqk994hilagk5cgv7vw9yzg-python3-3.7.3/lib/python3.7/pathlib.py", line 1209, in write_bytes
    return f.write(view)
OSError: [Errno 28] No space left on device