[3.13] Docs: fix spelling of "test case" in unittest documentation (GH-134137) (#134139)

Docs: fix spelling of "test case" in `unittest` documentation (GH-134137)
(cherry picked from commit b41d79c)

Co-authored-by: Clifford Gama <53076065+cliff688@users.noreply.github.com>

Author: miss-islington

Doc/library/unittest.rst

@@ -110,7 +110,7 @@ Here is a short script to test three string methods::
       unittest.main()
 
 
-A testcase is created by subclassing :class:`unittest.TestCase`.  The three
+A test case is created by subclassing :class:`unittest.TestCase`.  The three
 individual tests are defined with methods whose names start with the letters
 ``test``.  This naming convention informs the test runner about which methods
 represent tests.