Split test_levels into multiple properties

[mattias-p]

Depends on:

• Test module registration #1335

Today if you want to override the severity of a single test message you have to
specify the severities of all the test messages. This is rather cumbersome.
Instead of having a single complex test_levels property, one property for each
message tag reported by a test module should be dynamically created. I.e. when
the Basic module is registered and it reports a B01_NO_CHILD message tag, a
test_levels.BASIC.B01_NO_CHILD property should be dynamically created.