Disable some tests if running on root.

[phprus]

On root user:

-------------------------------------------------------------------------------
fs.op.permissions - permissions
-------------------------------------------------------------------------------
/home/.../test/filesystem_test.cpp:2440
...............................................................................

/home/.../test/filesystem_test.cpp:2448: FAILED:
  CHECK_THROWS_AS( fs::resize_file("foo", 1024), fs::filesystem_error )
because no exception was thrown where one was expected:

/home/.../test/filesystem_test.cpp:2449: FAILED:
  CHECK( fs::file_size("foo") == 512 )
with expansion:
  1024 (0x400) == 512 (0x200)

-------------------------------------------------------------------------------

[gulrak]

Thank you, this was an issue on the BSD CI runs as well.