[test] Add a FIXME test for stop-command-source-on-error

Modifying the interpreter settings is tricky because they don't take
effect until we create a new command interpreter, which should be merely
an implementation detail. This leads to confusing and unexpected
scenarios.

This adds a test cases with FIXMEs for some of the odd scenarios I
encountered. I didn't XFAIL the test because I don't think there's a way
to get an unexpected PASS if any of the commands succeeds and splitting
up the file in multiple tests seems excessive.

llvm-svn: 371259