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
* Avoid issue with breakpoint ordering (see comment)
* Consistently abort the debugger and wait for the `executeTask`
* Make `Get-Process` test cross-platform (and execute the right line)

Author: andyleejordan

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

@@ -15,8 +15,10 @@ 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 $PID
 	Write-Output "Done"
 }
 
 Test-Variables
+# NOTE: If a line is added to the function above, the line numbers in the
+# associated unit tests MUST be adjusted accordingly.