Introduce context manager for restoring environment in tests

Many tests modify the environment in some steps and need to restore it
afterwards.
To avoid missing this the contextmanager does that automatically.

Author: Flamefire

test/framework/easyblock.py

@@ -2640,7 +2640,7 @@ def test_extensions_sanity_check(self):
         eb = EB_toy(EasyConfig(self.eb_file))
         eb.silent = True
         write_file(os.path.join(eb.installdir, 'any_file'), '')
-        with self.mocked_stdout_stderr():
+        with self.mocked_stdout_stderr(), self.saved_env():
             eb.sanity_check_step()
         logtxt = read_file(eb.logfile)
         self.assertRegex(logtxt, 'Running .*command.*echo "toy output"')