Add a simple test of when_imagej_starts callback

This test checks that functions registered via
when_imagej_starts get called as part of imagej.init.

conftest.py

@@ -43,6 +43,8 @@ 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"))
+
     mode = "headless" if headless else "interactive"
     ij = imagej.init(ij_dir, mode=mode, add_legacy=legacy)
 

tests/test_callbacks.py

@@ -0,0 +1,7 @@
+def test_when_imagej_starts(ij):
+    """
+    The ImageJ2 gateway test fixture registers a callback function via
+    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)