Skip tests in TrustsV3TestJSON with immutable user source

Adds a simple condition in skip_checks to skip the entire test class
if the environment is configured to have an immutable user source. If
so, a skipException is raised and the entire class is not executed.

Partial-Bug: 1777047

Change-Id: I8c1c1df25401157c667bce97d4995f738eec8e05
1 file changed