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
* Pass actual script path to `BreakpointDetails.Create` so the debug
  service has a valid value for its hashtable key
* Call `Close()` instead of `Dispose()` (which changed) on
  `PowerShellContextService`
* Make assertion in `AssertDebuggerStopped` case-insensitive
* Increase cancellation token source timeouts to 10 seconds
* Avoid waiting for PowerShell context state change that never happens
  and isn't required
* Delete unused fields or local variables
* Fix incorrect assumption of breakpoint's index
* Make `Get-Process` test cross-platform (and execute the right line)

Author: andyleejordan

test/PowerShellEditorServices.Test.Shared/Debugging/VariableTest.ps1

@@ -15,7 +15,7 @@ function Test-Variables {
     $nullString = [NullString]::Value
     $psObjVar = New-Object -TypeName PSObject -Property @{Name = 'John';  Age = 75}
     $psCustomObjVar = [PSCustomObject] @{Name = 'Paul'; Age = 73}
-    $procVar = Get-Process system
+    $procVar = Get-Process -PID 0 # NOTE: PID 0 should always exist
 	Write-Output "Done"
 }