Move strategies to test subfolder

tests/strategy/strats/failing_strategy.py

@@ -0,0 +1,9 @@
+# The strategy which fails to load due to non-existent dependency
+
+import nonexiting_module  # noqa
+
+from freqtrade.strategy.interface import IStrategy
+
+
+class TestStrategyLegacy(IStrategy):
+    pass