internal/getproviders: Tests for MultiSource

Due to other pressures at the time this was implemented, it was tested
only indirectly through integration tests in other packages.

This now introduces tests for the two main entry points on the
MultiSource, along with its provider-address pattern matching logic.

This does not yet include thorough tests for
ParseMultiSourceMatchingPatterns, because that function still needs some
adjustments to do the same case folding as for normal provider address
parsing, which will follow in a latter commit along with suitable tests.

With that said, the tests added here do _indirectly_ test the happy path
of ParseMultiSourceMatchingPatterns, so we have some incomplete testing
of that function in the meantime.

Author: apparentlymart