Support test plans that specify a command assertion exception that does not include an assertion priority prefix

[mcking65]

This is a test writing simplification proposal that is related to #1045. It might be a good idea to implement both #1045 and this request in a single PR.

Currently, assertion exceptions in the *-commands.csv files are required to have a priority prefix of n: where n is 0, 1, 2, or 3. This proposal is to support omitting the priority prefix in the assertionExceptions column of *-commands.csv in situations where the test author wants the priority specified in tests.csv or assertions.csv to be assigned to the assertion.

For example, the priority of assertions for switching screen reader mode is specified in assertions.csv. It is typically 2. It is redundant to require the test author to specify it again in the commands file. Requiring the author to repeat the priority in the commands file creates opportunity for accidental inconsistency across AT or even among different commands for a specific AT.

The current priority precedence defined in the V2 format would not change. That is:

1. The priority specified in commands.csv is assigned if specified.
2. If priority is not specified in commands.csv, the priority specified in tests.csv is assigned.
3. If priority is not specified in tests.csv, the priority specified in assertions.csv is assigned.

[ccanash]

Hi @mcking65 can you please confirm this works as expected? Please see comment in #1053