[3.14] gh-156939: Add tests on the PyUnicode C API (#159533) (#159539)

[3.15] gh-156939: Add tests on the PyUnicode C API (#159533)

* gh-142217: Add tests on the deprecated _Py_Identifier C API (#157638)

(cherry picked from commit 21403af)

* gh-156939: Add tests on the PyUnicode C API (#157798)

Add tests on functions:

* PyUnicode_CHECK_INTERNED()
* PyUnicode_Check()
* PyUnicode_CheckExact()
* PyUnicode_Equal()
* PyUnicode_GET_LENGTH()
* PyUnicode_IS_ASCII()
* PyUnicode_IS_COMPACT()
* PyUnicode_IS_COMPACT_ASCII()
* PyUnicode_KIND()
* PyUnicode_MAX_CHAR_VALUE()
* PyUnicode_READ()
* PyUnicode_READ_CHAR()
* PyUnicode_WRITE()

Changes:

* Add test on PyUnicode_FromOrdinal(0).
* Require _testcapi module in test_capi.test_unicode. Since _testcapi
  is specific to CPython, remove @support.cpython_only.

(cherry picked from commit e8f6448)
(cherry picked from commit bcbe9bc)

Author: vstinner