[fix_jira_144] Missing Methods detection test fix

HashMap -> TreeMap in ImplementedMethodsHelper for consistent collection index order in the keySet, since TreeMap uses the natural order for its keys by default (whereas HashMap doesn't).

This should fix the TestMissingMethodsError#testCollMissingPrintLnFlushX2 error in the adl-backend.

Bug happening with Bamboo, expecting error on s[0], but happening with s[1], since HashMap
keySet are not ordered. The Map is used in CPLChecker#postParseChecks by getting the keySet, transforming it as an Array, and getting its first element. However the Map was created in the ImplementedMethodsHelper, hence this commit.