In three occasions tests are “easy” to write:
The specifications of the language are a fine source for many tests. For instance the specification of integer literals show several cases to exercise.
If your compiler crashes or fails, before even trying to fix it, include the test case in your test suite.
If you are developing a component for the compiler, you can certainly feel the weak points. Immediately write a test for these.
See Testing student-made compilers, for many hints on what tests you need to write.