[lldb] Add assertState function to the API test suite

[lldb] Add assertState function to the API test suite

Add a function to make it easier to debug a test failure caused by an
unexpected state.

Currently, tests are using assertEqual which results in a cryptic error
message: "AssertionError: 5 != 10". Even when a test provides a message
to make it clear why a particular state is expected, you still have to
figure out which of the two was the expected state, and what the other
value corresponds to.

We have a function in lldbutil that helps you convert the state number
into a user readable string. This patch adds a wrapper around
assertEqual specifically for comparing states and reporting better error
messages.

The aforementioned error message now looks like this: "AssertionError:
stopped (5) != exited (10)". If the user provided a message, that
continues to get printed as well.

Differential revision: https://reviews.llvm.org/D127355