Use descriptive attribute name for callback test

Use a more descriptive attribute name and value for
when_imagej_starts() test.

conftest.py

@@ -43,9 +43,7 @@ def ij(request):
     legacy = request.config.getoption("--legacy")
     headless = request.config.getoption("--headless")
 
-    imagej.when_imagej_starts(
-        lambda ij: setattr(ij, "_when_imagej_starts_result", "success")
-    )
+    imagej.when_imagej_starts(lambda ij: setattr(ij, "_testing", True))
 
     mode = "headless" if headless else "interactive"
     ij = imagej.init(ij_dir, mode=mode, add_legacy=legacy)

tests/test_callbacks.py

@@ -4,4 +4,4 @@ def test_when_imagej_starts(ij):
     when_imagej_starts, which injects a small piece of data into the gateway
     object. We check for that data here to make sure the callback happened.
     """
-    assert "success" == getattr(ij, "_when_imagej_starts_result", None)
+    assert True == getattr(ij, "_testing", True)