[lldb/test] Catch invalid calls to expect()

Add preconditions to `TestBase.expect()` that catch semantically invalid calls
that happen to succeed anyway. This also fixes the broken callsites caught by
these checks.

This prevents the following incorrect calls:

1. `self.expect("lldb command", "some substr")`
2. `self.expect("lldb command", "assert message", "some substr")`

Differential Revision: https://reviews.llvm.org/D88792