Fix fast reader tests and warnings for "win32" - #6

Description

Fixes some issues found in astropy#15235 (comment)

1. The "win32" job uses a 64 bit compiler and as such passes the double precision test, just shows somewhat deviant behaviour on warnings.
2. A construct such as https://github.com/astropy/astropy/blob/53188c39a23c33b72df5850ec59e31886f84e29d/astropy/io/ascii/tests/test_c_reader.py#L1401-L1410
   would have never worked for 0 warnings, as pytest.warns() already raises an error in that case (fixed with pytest.warns(Warning)).