Long ago, dbtest could be run with no args, and all the suites inside dbtest would run. This was how buildbot tested dbtest.
After the introduction of Evergreen, we switched the testing of dbtest to run each suite with a separate invocation of dbtest, in order to parallelize the run and make it faster. This means we lost test coverage of running dbtest with no arguments, and thus today dbtest actually fails if you run it with no arguments, since some tests leave behind state that cause subsequent tests to fail.
Since we have no plans to ever add back an Evergreen suite that runs dbtest with no arguments, we should make it an error to operate dbtest in this way, to prevent engineer confusion in the future. The error message should provide guidance as to how to run dbtest correctly (using resmoke will run dbtest separately for each suite, for example).