gh-158942: Use more unique variable names in Py_MIN/MAX/ABS() (#158969)

If Py_MIN/MAX/ABS() macros are called on an existing "_x" or "_y"
variable name, the external variable is used instead of macro local
variable. So use more unique names in the macros to avoid
the issue.

Add test_min_max_abs_macros() to _testlimitedcapi.