gh-151162: Increase test coverage for string formatting

[manoj-marimuthu]

In Lib/test/test_str.py, string formatting is tested for nan and inf but not for -inf. -inf could be the most important among them (since we have to check if '-' formats correctly). The following PR adds 2 test cases for -inf with %f and %F.

Existing:

self.assertEqual('%f' % NAN, 'nan')
self.assertEqual('%F' % NAN, 'NAN')
self.assertEqual('%f' % INF, 'inf')
self.assertEqual('%F' % INF, 'INF')

Added:

self.assertEqual('%f' % -INF, '-inf')
self.assertEqual('%F' % -INF, '-INF')

• Issue: Increase test coverage for string formatting #151162

[StanFromIreland]

This is already covered in test_float.RoundTestCase.test_format_specials, and I don't think the second case is particularly special here.

[bedevere-app]

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