To run all tests make sure you have deltachat-ios target selected and long press the Run button (aka play button) at the top to switch to Test.
To run individual tests navigate to the test function you want to run (inside the DcTests.swift file) and click the Test button on the left of the first line of the test function (in the line number column).
To create new unit tests add a function annotated with the @Test macro inside the DcTests struct. See the Swift Testing docs for how to write tests.