Correct type hint on Test.execute() (#975)

The function docstring says `test_start` can be "Either a trigger phase for starting the test, or a function that returns a DUT ID.`

Unfortunately, when you provide the latter, Mypy is unhappy:
```
Argument "test_start" to "execute" of "Test" has incompatible type "Callable[[], str]"; expected "Union[PhaseDescriptor, Callable[..., Optional[PhaseResult]], None]"  [arg-type]
```

This commit updates the type hint of the parameter to be a Union that reflects the docstring.

Author: lalten

openhtf/core/test_descriptor.py

@@ -275,7 +275,7 @@ def abort_from_sig_int(self) -> None:
         self._executor.abort()
 
   def execute(self,
-              test_start: Optional[phase_descriptor.PhaseT] = None,
+              test_start: Optional[Union[phase_descriptor.PhaseT, Callable[[], str]]] = None,
               profile_filename: Optional[Text] = None) -> bool:
     """Starts the framework and executes the given test.