Custom targets tests

add make target tests_run to conveniently run run_tests.sh

PS: im open to suggestions to names of the targets, currently I tried to stick with similar to existing binaries (htmtest, testeverything and run_tests), thus tests_htm, tests_everything and tests_run. 
Or we could be more creative, and use more descriptive names: tests_htm, tests_cpp, tests_py ??