tests: Document the unique index needed for the tests table

This is needed for "INSERT OR REPLACE INTO tests" to work correctly with
run-tests.py -L.

Signed-hostap: Jouni Malinen <j@w1.fi>

tests/hwsim/README

@@ -181,6 +181,7 @@ CREATE TABLE results (test,result,run,time,duration,build,commitid);
 CREATE INDEX results_idx ON results (test);
 CREATE INDEX results_idx2 ON results (run);
 CREATE TABLE tests (test,description);
+CREATE UNIQUE INDEX tests_idx ON tests (test);
 CREATE TABLE logs (test,run,type,contents);
 CREATE INDEX logs_idx ON logs (test);
 CREATE INDEX logs_idx2 ON logs (run);