Improve the test function for pow

[ppmfloss]

Currently in the powtest function in Lib/test/test_pow.py file we are not testing the results of calling pow with negative exponents.
This PR will add appropriate assert statements to test the results of calling pow with negative exponents.

Linked PRs

• GH-102341: Improve the test function for pow #102342
• [3.11] GH-102341: Improve the test function for pow (GH-102342) #102446
• [3.10] GH-102341: Improve the test function for pow (GH-102342) #102447

[terryjreedy]

Thanks.