[mlir] [test] Fix unittests in standalone builds (#120910)
Fix the logic used to run unit tests to account for `llvm_gtest` targets
being installed, since 91b3ca39667b6341a8c1983a1467fae14b58318b. This
involves removing a rule that would cause a duplicate `llvm_gtest`
target being created, and updates the method for determining whether
unittests can be run to checking whether the target is present, rather
than the source directory (that is no longer actually necessary).
