test: Fix tagguessertest lookup error with ctest.
For whatever reason, running plain ctest or even "ninja test" on a testing-enabled does not succeed in finding tagguessertest to run it, probably because this test is located in the build directory's /bin subfolder and the test metadata is incorrect. However I was able to solve this just by using the existing ECM macro to add a single test (https://api.kde.org/ecm/module/ECMAddTests.html) directly, so that's what I did. ctest now runs and passes both tests fine again.
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