[lldb] Don't search for system specific strings in assert.test

Commit 6bc1e69d changed the search string
to also check for the generated strings that surround the plain assert:

    Assertion `false && "lldb-test assert"' failed
                                         ^^^^^^^^^

This causes the test to fail on setups where the generated assert message
looks different. E.g., on macOS the generated message looks like this:

    Assertion failed: (false && "lldb_assert failed"), function lldb_assert

This reverts the old behaviour of just checking for the actual string we
have inside LLDB.