8076543: Add @modules as needed to the langtools tests
Reviewed-by: jjg, shurailine

8062348: langtools tests should close file manager (group 1)
Reviewed-by: darcy

7142086: performance problem in Check.checkOverrideClashes(...)
Summary: Code in Check.checkOverrideClashes() causes too many calls to MethodSymbol.overrides
Reviewed-by: jjg
Contributed-by: jan.lahoda@oracle.com