java: Fix regression _find_rw_exec_dir() failing to handle ro dirs

[Jongy]

Fixes a regression from 6360449, and improves the tests
so we'd catch it next time :)

[Jongy]

I will also run one negative test - undo the fix and ensure that the new test fails.

[Jongy]

Commenting-out the fixing code in the plugin - the tests fail as expected.

tests/test_java.py::test_java_noexec_or_ro_dirs[on_host-noexec] PASSED                                                                                                                                                               [ 25%]
tests/test_java.py::test_java_noexec_or_ro_dirs[on_host-ro] FAILED                                                                                                                                                                   [ 50%]
tests/test_java.py::test_java_noexec_or_ro_dirs[in_container-noexec] PASSED                                                                                                                                                          [ 75%]
tests/test_java.py::test_java_noexec_or_ro_dirs[in_container-ro] FAILED

Traceback (most recent call last):
  File "/Granulate/gprofiler/gprofiler/profilers/profiler_base.py", line 132, in _wait_for_profiles
    result = future.result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 437, in result
    return self.__get_result()
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 389, in __get_result
    result = self.fn(*self.args, **self.kwargs)                                                                                                                                                                                    [98/1807]
  File "/Granulate/gprofiler/gprofiler/profilers/java.py", line 1211, in _profile_process
    with AsyncProfiledProcess(
  File "/Granulate/gprofiler/gprofiler/profilers/java.py", line 498, in __init__
    self._ap_dir_base = self._find_rw_exec_dir()
  File "/Granulate/gprofiler/gprofiler/profilers/java.py", line 545, in _find_rw_exec_dir
    mkdir_owned_root(full_dir)
  File "/Granulate/gprofiler/gprofiler/utils/fs.py", line 91, in mkdir_owned_root
    os.mkdir(path, mode=mode)
OSError: [Errno 30] Read-only file system: '/proc/1/root/tmp/pytest-of-root/pytest-22/test_java_noexec_or_ro_dirs_on1/tmpfs/gprofiler_tmp'