New semantic analyzer: enable function tests (#6611)

This changes a false positive to a false negative, which is
perhaps a reasonable tradeoff for now.  I filed an issue about
the false negative (#6610).

Author: JukkaL

mypy/test/hacks.py

@@ -10,7 +10,6 @@
     'check-async-await.test',
     'check-expressions.test',
     'check-flags.test',
-    'check-functions.test',
     'check-incremental.test',
     'check-literal.test',
     'check-overloading.test',

test-data/unit/check-functions.test

@@ -1394,6 +1394,7 @@ else:
     def f(x: str) -> None: pass # E: Incompatible redefinition (redefinition with type "Callable[[str], None]", original type "Callable[[int], None]")
 
 [case testConditionalFunctionDefinitionUsingDecorator3]
+# flags: --new-semantic-analyzer
 from typing import Callable
 
 def dec(f) -> Callable[[int], None]: pass
@@ -1402,11 +1403,11 @@ x = int()
 if x:
     def f(x: int) -> None: pass
 else:
-    # TODO: This should be okay.
-    @dec # E: Name 'f' already defined on line 7
+    @dec
     def f(): pass
 
 [case testConditionalFunctionDefinitionUsingDecorator4]
+# flags: --new-semantic-analyzer
 from typing import Callable
 
 def dec(f) -> Callable[[int], None]: pass
@@ -1415,8 +1416,8 @@ x = int()
 if x:
     def f(x: str) -> None: pass
 else:
-    # TODO: We should report an incompatible redefinition.
-    @dec # E: Name 'f' already defined on line 7
+    # TODO: Complain about incompatible redefinition
+    @dec
     def f(): pass
 
 [case testConditionalRedefinitionOfAnUnconditionalFunctionDefinition1]