Make coverage work without test defs.

Change-Id: I946df038e97dc5c2f40a4610d4076e13ab6bde37
diff --git a/testrunner/coverage/__init__.py b/testrunner/coverage/__init__.py
new file mode 100644
index 0000000..4be2078
--- /dev/null
+++ b/testrunner/coverage/__init__.py
@@ -0,0 +1 @@
+__all__ = ['coverage', 'coverage_targets', 'coverage_target']