Enable strict optional for more test files (5) (#15602)

Followup to #15586

Co-authored-by: Shantanu <[email protected]>

Author: cdce8p

mypy/test/testcheck.py

@@ -51,7 +51,6 @@
 
 # TODO: Enable strict optional in test cases by default. Remove files here, once test cases are updated
 no_strict_optional_files = {
-    "check-expressions.test",
     "check-functions.test",
     "check-generic-subtyping.test",
     "check-generics.test",