Support trio_run = trio_asyncio ini option.

`trio_asyncio` as another mode of the `trio_run` ini file option allows
us to run tests with an `asyncio` loop implicitly available to all async
tests.

python-trio#71

docs/source/reference.rst

@@ -393,7 +393,8 @@ such as via :ref:`guest mode <trio:guest-mode>`, it can be used with
 pytest-trio as well.  Setting ``trio_run`` in the pytest configuration
 makes your choice the global default for both tests explicitly marked
 with ``@pytest.mark.trio`` and those automatically marked by Trio mode.
-``trio_run`` presently supports ``trio`` and ``qtrio``.
+``trio_run`` presently supports ``trio``, ``qtrio``, and
+``trio_asyncio``.
 
 .. code-block:: ini
 

pytest_trio/_tests/test_trio_mode.py

@@ -55,6 +55,10 @@ def run(*args, **kwargs):
 """
 
 
+# Fake trio_asyncio module.
+trio_asyncio_text = qtrio_text
+
+
 def test_trio_mode_and_qtrio_run_configuration(testdir):
     testdir.makefile(".ini", pytest="[pytest]\ntrio_mode = true\ntrio_run = qtrio\n")
 
@@ -116,6 +120,27 @@ async def test_fake_qtrio_used():
     result.assert_outcomes(passed=1)
 
 
+def test_trio_asyncio_just_run_configuration(testdir):
+    testdir.makefile(".ini", pytest="[pytest]\ntrio_run = trio_asyncio\n")
+
+    testdir.makepyfile(trio_asyncio=trio_asyncio_text)
+
+    test_text = """
+    import pytest
+    import trio_asyncio
+    import trio
+
+    @pytest.mark.trio
+    async def test_fake_trio_asyncio_used():
+        await trio.sleep(0)
+        assert trio_asyncio.fake_used
+    """
+    testdir.makepyfile(test_text)
+
+    result = testdir.runpytest()
+    result.assert_outcomes(passed=1)
+
+
 def test_invalid_trio_run_fails(testdir):
     run_name = "invalid_trio_run"
 

pytest_trio/plugin.py

@@ -30,7 +30,7 @@ def pytest_addoption(parser):
     )
     parser.addini(
         "trio_run",
-        "what runner should pytest-trio use? [trio, qtrio]",
+        "what runner should pytest-trio use? [trio, qtrio, trio_asyncio]",
         default="trio",
     )
 
@@ -515,10 +515,14 @@ def choose_run(config):
         import qtrio
 
         run = qtrio.run
+    elif run_string == "trio_asyncio":
+        import trio_asyncio
+
+        run = trio_asyncio.run
     else:
         raise ValueError(
             f"{run_string!r} not valid for 'trio_run' config."
-            + "  Must be one of: trio, qtrio"
+            + "  Must be one of: trio, qtrio, trio_asyncio"
         )
 
     return run