You can choose to run a group of tests by editing the build scheme. To edit the build scheme, click on the scheme in the toolbar in Xcode, and then click on Edit Scheme...:

Then, select Test, and expand the test suite by clicking on the small triangle. On the right-hand side, there is a column called Test:

The selected scheme only runs the tests that are checked. By default, all the tests are checked, but you can uncheck some tests if you need to. But don't forget to check all the tests again when you are finished.

As an alternative, you can add a build scheme for a group of tests that you want to run regularly without running all tests.

But as mentioned earlier, you should run the complete test suite as often as possible.