t4254: modernize tests

t4254: modernize tests

- Don't start tests with 'test $? = 0' to catch preparation done
  outside the test_expect_success block.

- Move writing the bogus patch and the expected output into the
  appropriate test_expect_success blocks.

- Use the test_must_fail helper instead of manually checking for
  non-zero exit code.

- Use the debug-friendly test_path_is_file helper instead of 'test -f'.

- No space after '>'.

Signed-off-by: SZEDER Gábor <redacted>
Signed-off-by: Junio C Hamano <redacted>