testing: Add TEST_TO_RUN to allow faster iteration when debugging.

People can set TEST_TO_RUN=test-name to run only a specific test.

This only works for tests that are set up to use run_test, which is currently
only the ones under automatic/, but I'm planning to convert system/ etc in a
followup.

pagespeed/automatic/system_test_helpers.sh

@@ -36,8 +36,9 @@
 #
 #
 # By default tests that are in separate files and run with run_test are run
-# asynchronously.  To disable this, for more predictable debugging, set the
-# environment variable RUN_TESTS_ASYNC to "off".
+# synchronously.  Run them asynchronously, set the environment variable
+# RUN_TESTS_ASYNC to "on".  The tests will probably flake if run this way, so
+# it's not recommended.
 #
 # Callers need to set SERVER_NAME, and not run this more than once
 # simultaneously with the same SERVER_NAME value.
@@ -124,6 +125,14 @@ cat > $WGETRC <<EOF
 user_agent = Mozilla/5.0 (X11; U; Linux x86_64; en-US) AppleWebKit/534.0 (KHTML, like Gecko) Chrome/6.0.408.1 Safari/534.0
 EOF
 
+# You can pass in TEST_TO_RUN=test-name to run only a specific test.  This is
+# intended for debugging, where you want to iterate on a single failing test.
+# It only works with tests that are set up to use run_test, which is currently
+# only the ones in automatic/.  Tests that haven't been converted to use
+# run_test currently always run.
+# TODO(jefftk): convert all system tests to use run_test and separate files.
+TEST_TO_RUN="${TEST_TO_RUN:-}"
+
 # Individual tests should use $TESTTMP if they need to store something
 # temporarily.  Infrastructure can use $ORIGINAL_TEMPDIR if it's ok with
 # parallel use.
@@ -227,6 +236,10 @@ BACKGROUND_TEST_NAMES=() # hash from pid to name of test
 function run_test() {
   local test_name=$1
 
+  if [ -n "$TEST_TO_RUN" ] && [ "$TEST_TO_RUN" != "$test_name" ]; then
+    return  # By default TEST_TO_RUN="" so normally we don't skip tests here.
+  fi
+
   if $RUN_TESTS_IN_BACKGROUND; then
     while [ $(jobs | wc -l) -gt $PARALLEL_MAX ]; do
       sleep .1  # Wait for background tasks to complete.