The unittests rely on a separately packaged version of gpgmejs, with the different classes and functions exposed. These tests and their input values can be found in gpgme/lang/js/test. They do not test the overall functionality, but the individual behaviour of the components.
The functionality tests, to be found in gpgme/lang/js/BrowserTestExtension, check the overall functionality of the standard packaged version of gpgmejs.
Most tests rely on a test gpg key to be available in gpg, which can be found at the bottom of this page, or as "testkey.sec" in the BrowserTestExtension's directory. Please import this key to your tested gpg installation, or adapt the input defined in tests/inputvalues.js if you want to use different values.