Bugfix for issue 15782: Remove needless functions from an io.ascii test helper file

[stefanarseneau]

EDIT: close #15839

resolving issue, fix #15782

replaced all calls of:
assert_equal() -> np.testing.assert_equal()
assert_almost_equal() -> np.testing.assert_allclose()
assert_true(a) -> assert a
in the following files:
astropy/io/ascii/tests/test_c_reader.py
astropy/io/ascii/tests/test_cds.py
astropy/io/ascii/tests/test_rst.py
astropy/io/ascii/tests/test_types.py
astropy/io/ascii/tests/test_read.py
astropy/io/ascii/tests/test_fixedwidth.py
astropy/io/ascii/tests/test_cds_header_from_readme.py

Description

This pull request is to address ...

Fixes #

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[neutrinoceros]

Thank you for your effort, however, please note that there is already an other PR opened to fix #15782 that we are already invested in reviewing (#15839).

[eerovaher]

It is true that there already is another pull request addressing the same issue, but there is little reason to believe it will ever be finished, so this shouldn't be closed as a duplicate. That being said, this needs to be rewritten to a large extent so that it would follow the updated instructions in #15782.