[lldb] Replace asserts on .Success() with assertSuccess()

Replace forms of `assertTrue(err.Success())` with `assertSuccess(err)` (added in D82759).

* `assertSuccess` prints out the error's message
* `assertSuccess` expresses explicit higher level semantics, both to the reader and for test failure output
* `assertSuccess` seems not to be well known, using it where possible will help spread knowledge
* `assertSuccess` statements are more succinct

Differential Revision: https://reviews.llvm.org/D119616