Fix error with wrong argument to gpaw-analyse-basis

This silly error snuck in when converting the test from subprocess-based to pure unittest.

@jensj Should something be done about integration-testing of GPAW CLI tools? In ASE it's always assumed that the ase command is available, and then there's a fixture called cli which allows issuing commands easily.

In GPAW it's a bit more sketchy because there are still some commands that are not subcommands of gpaw. But they're still installed by pip, right? So they should be there in a valid installation.

Fixes: #324 (closed)