Add a Thing for API testing

This may not be in the codebase forever, but it's a very useful
sanity check to have during development.

The current implementation is clearly not a performance or
security risk.

src/openflexure_microscope_server/things/test.py

@@ -0,0 +1,31 @@
+"""
+OpenFlexure Microscope API test Thing
+
+This Thing is intended only for use testing out the API and client(s).
+"""
+from labthings_fastapi.thing import Thing
+from labthings_fastapi.decorators import thing_action, thing_property
+from labthings_fastapi.dependencies.invocation import CancelHook, InvocationLogger
+
+
+class APITestThing(Thing):
+    _counter: int = 0
+
+    @thing_property
+    def counter(self) -> int:
+        return self._counter
+
+    @thing_action
+    def count_slowly(
+        self,
+        cancel: CancelHook,
+        logger: InvocationLogger,
+        n=100,
+        dt=0.1,
+    ) -> None:
+        for i in range(n):
+            self._counter = i
+            print(f"counted to {i}")
+            logger.info(f"Counted to {i}")
+            cancel.sleep(dt)
+        self._counter = n