Rename submissions api file
Unclobber the submission API tests.

When two test suites have the same name, they overwrite each others'
teardown and setup methods, and other defined methods if there are any.

Exercism uses minitest v5.x, which uses the Minitest::Test syntax. This
is incompatible with the json_expressions gem at the moment, so I
converted the hybrid Minitest::Unit::TestCase to reference the old
version of minitest.
Turn off warnings in submission api test
Consolidate Submission API tests

Match the file structure of the rest of the project.