Add tools/run_tests/run_lcov.sh

This is a tool for collecting coverage reports from our tests and
combining them with the open-source tool 'lcov'.
diff --git a/.gitignore b/.gitignore
index 5202b53..192b55f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,4 +3,5 @@
 libs
 objs
 *.pyc
+*.gcno