stbt.py: `mock_get_config` returns same types as real `get_config`
stbt.py: `mock_get_config` returns same types as real `get_config`

(in `test_build_templatematch_params_detects_undefined`).

This doesn't really affect the test; it's just for consistency with
the real `get_config`.

martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,wmanley/stb-tester