gh-106316: Remove pytime.h header file (#106317)

Remove the "cpython/pytime.h" header file: it only contained private
functions. Move functions to the internal pycore_time.h header file.

Move tests from _testcapi to _testinternalcapi. Rename also test
methods to have the same name than tested C functions.

No longer export these functions:

* _PyTime_Add()
* _PyTime_As100Nanoseconds()
* _PyTime_FromMicrosecondsClamp()
* _PyTime_FromTimespec()
* _PyTime_FromTimeval()
* _PyTime_GetPerfCounterWithInfo()
* _PyTime_MulDiv()