tests: Add tests for error states in r.mapcalc

[wenzeslaus]

The test with the rand function fails with higher number of threads (4 and above in tests).

[wenzeslaus]

Linux ends with -11, so segfault (same as my local Linux test):

FAILED raster/r.mapcalc/tests/r_mapcalc_nprocs_test.py::test_rand_no_explicit_seed_setting[10] - AssertionError: assert -11 == 1
 +  where -11 = ToolResult(returncode=-11, stdout='', stderr='ERROR: Pseudo-random number generator not seeded\n').returncode

Windows produces 1 for return code (desired), but does not produce an error message:

 FAILED raster/r.mapcalc/tests/r_mapcalc_nprocs_test.py::test_rand_no_explicit_seed_setting[0] - AssertionError: assert 'not seeded' in ''
 +  where '' = ToolResult(returncode=1, stdout='', stderr='').stderr

macOS runs the tests and they pass.

[wenzeslaus]

We can merge this (assuming CI passes) and #6480 can delete the @pytest.mark.skip line.