8181867: [tests] Reorganize EchoHandlers
Summary: This fix reorganize some test files and rename some test classes. Several classes named EchoHandler in the unnamed package are renamed to make it clear what classes (and sources) tests that use these EchoHandler implementations effectively depend on.
Reviewed-by: chegar

8170648: Move java.net.http package out of Java SE to incubator namespace
Reviewed-by: chegar, dfuchs, michaelm, prappo, skuksenko
Contributed-by: Chris Hegarty <chris.hegarty@oracle.com>, Daniel Fuchs <daniel.fuchs@oracle.com>, Michael McMahon <michael.x.mcmahon@oracle.com>, Pavel Rappo <pavel.rappo@oracle.com>, Sergey Kuksenko <sergey.kuksenko@oracle.com>, anthony.vanelverdinghe@gmail.com