Add diag for inputflinger tests

In some cases diag may give us better error reporting. We can't add diag
to production code because it would slow down the code, but having it on
tests shouldn't have any impact.

Bug: 378308551
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Flag: TEST_ONLY
Change-Id: I91349ee7e880685519690a47580a18f240d00b9f