There's five tests relating to adding or dropping keys in ddl_tests.php that appear to be passing by accident.

testAddUniqueKey
testAddForeignUniqueKey
testAddForeignKey
testDropKey
testDropForeignKey

When they add keys the command is being silently dropped because a key with that name already exists.

Spotted while looking at http://tracker.moodle.org/browse/MDL-36930 (SQLite unit test failure: duplicate keys when adding Foreign Keys)

Note the tests don't actually test for the presence (or absence) of the keys they add or drop, yet there's a test in the same file called testFindKeyName that (mostly) does exactly that. Perhaps that could be re-used to make these a bit stricter?