During normal site operation, namespaces of disabled modules are not registered with the autoloader. This means if a module tries to instantiate a class provided by a disabled module, an error is (correctly) thrown.
However, during testing, simpletest.module registers the namespaces of all disabled modules in order to find their tests. This means that if a module has the error mentioned above, then it will not be discovered by the test.
In #1780396-54: Namespaces of disabled modules are registered during test runs, @Berdir suggested to fix this by limiting simpletest to only register the Tests subnamespace of each module. However, in that issue, @sun raised concerns about that making unit test writing too cumbersome (since in unit tests, there's no concept of enabling modules).