Test.isRunningTest() is meant to be used in the class that the test is testing not in the test class itself. See the docs here for more information on that method.
Returns true if the currently executing code was called by code
contained in a test method, false otherwise. Use this method if you
need to run different code depending on whether it was being called
from a test.
Found a helpful article here that goes over examples of when to use the method.
Other Usage scenarios:
1) To ensure the trigger doesn't execute the batch if
Test.IsRunningTest() is true, and then test the batch class with it's
own test method.
2) Testing callouts - in your callout code you check to see if you're
executing within a unit test context by checking Test.isRunningTest()
and instead of getting your callout response from an
HttpResponse.send() request, you return a pre-built test string