Support test plans that specify a command assertion exception for an assertion that is not listed in tests.csv

[mcking65]

This is a test writing simplification proposal raised in the description of #1039 that we agreed not to implement in #1042.

If the author of a test plan wants to add one or more assertions to a subset of commands that are assigned to a test, and if the author wants those assertions to be at the end of the sequence of assertions, then support specifying one or more assertions in the assertionExceptions column of *-commands.csv without specifying those assertions as 0 priority assertions in tests.csv. Thus, the list of assertions that are included for a command includes:

1. All assertions listed in tests.csv in the order listed in tests.csv that have a non-0 priority in both tests.csv and commands.csv.
2. All the assertions listed for the command in commands.csv in the order listed in commands.csv where:
   1. Are not present in tests.csv
   2. Have a non-0 priority in commands.csv