[trace][intelpt] fix some test failures

Minor fixes needed and now `./bin/lldb-dotest -p TestTrace` passes
correctly.

- There was an incorrect iteration.
- Some error messages changed.
- The way repeat commands are handled changed a bit, so I had to create
a new --continue arg in "thread trace dump instructions" to handle this
correctly.

Differential Revision: https://reviews.llvm.org/D122023