run_unittests: fix unused variable warnings

We don't actually want to do anything with the open()ed file, just
immediately close it.

The CalledProcessError doesn't have its return returncode checked
here, even though other code with the same type of context manager does.

unittests/allplatformstests.py

@@ -538,7 +538,7 @@ class AllPlatformTests(BasePlatformTests):
 
     def test_implicit_forcefallback(self):
         testdir = os.path.join(self.unit_test_dir, '96 implicit force fallback')
-        with self.assertRaises(subprocess.CalledProcessError) as cm:
+        with self.assertRaises(subprocess.CalledProcessError):
             self.init(testdir)
         self.init(testdir, extra_args=['--wrap-mode=forcefallback'])
         self.new_builddir()
@@ -3605,7 +3605,7 @@ class AllPlatformTests(BasePlatformTests):
         cmakefile = Path(testdir) / 'subprojects' / 'sub2' / 'CMakeLists.txt'
         self.init(testdir)
         self.build()
-        with cmakefile.open('a', encoding='utf-8') as f:
+        with cmakefile.open('a', encoding='utf-8'):
             os.utime(str(cmakefile))
         self.assertReconfiguredBuildIsNoop()