Some unit tests are not getting run properly

It was recently discovered that a [tenant related unit test](https://github.com/apple/foundationdb/blob/8b0f53caf167c5bec46be4ca7412a08b2adedd6f/fdbclient/Tenant.cpp#L25) had a bug in it. This bug was not being detected in our normal correctness runs because it doesn't seem to be registered in the list of unit tests.

Based on the symptoms, it seems as if the build may have been optimizing out the test. In this case, the test was the only code inside of the file, and I was able to resolve the issue by moving an unrelated function definition from Tenant.h to Tenant.cpp.

Start with fdbclient/Tenant.cpp and Tenant.h, then inspect how unit tests are registered and included in normal correctness runs. Reproduce the missing tenant test and trace the build behavior; done means the test is reliably discovered and its bug is caught by the correctness run.

Written by the indexing model from the issue text.