maint: overhaul the format / resolution / unit test

[Stefan Appelhoff]

We have one parametrized test that covers many cases:

pybv/pybv/tests/test_bv_writer.py

Lines 226 to 229 in 05c5c63

 @pytest.mark.parametrize("format", ["binary_float32"]) 
 @pytest.mark.parametrize("resolution", resolutions) 
 @pytest.mark.parametrize("unit", ["V", "mV", "uV", "µV", "nV"]) 
 def test_unit_resolution(format, resolution, unit): 

the test should be split up into smaller and more targeted tests, including explicit cases that should fail, and explicit cases that should pass.

See: #62 (comment)