The filecheck directory contains compiler tests
The MLIR tests use lit (LLVM integrated tester).
lit allows specifying tests in-line in *.mlir files (using a RUN ... line).
This is used to run FileCheck tests
that check MLIR lowerings/conversions, but can also be used to simply verify that an *.mlir file parses correctly.
This setup seems to have been originally designed for regression tests in LLVM, so see Regression Test Structure for information on how this works.
There is a "check-heco" target in the CMake file which can be used to run these tests.
This will search for *.mlir files and try to run then.
Note that the test happen during the build phase of the CMake process, there is no way of running these targets.