[lldb][Test] Replace expect() with expect_expr() in TestNamespaceLookup.py

This will be useful in preparation for some reshuffling
of assertions in this file since we won't have to
adjust the persitent variable names during the process.

sed commands:
```
s/expect("expr -- /expect_expr("/g
s/startstr="(int) [$0-9]* = /result_type="int", result_value="/g
```

**Testing**

* API tests still pass

Differential Revision: https://reviews.llvm.org/D132271