Fix all DebugServiceTests

A number of items needed to be fixed:

* `Path.Combine` instead of `Path.Join` because of .NET changes
* `ConfigureAwait(false)` because of our rule to always include that
* Passing actual script path to `BreakpointDetails.Create` so the debut
  service has a valid value for its hashtable key
* Calling `Close()` instead of `Dispose()` (which changed) on
  `PowerShellContextService`
* Making assertion in `AssertDebuggerStopped` case-insensitive

Author: andyleejordan