Proposal: Rename Active Test Files to .test.

[astorm]

As preparation for a possible move from tape to tap, we should rename any test file that contains actual tests (but leave the _ include files or "other not TAP tests" alone) to include the .test.js extension. In other word run something like

% mv test/agent.js test/agent.test.js

for every file that has TAP tests. Once this renaming is complete we'll want to change the collection of test files logic intest/test.js (not renamed because it's not a TAP file) to only run *.test.js files.

[trentm]

Another benefit (IMO) is having a file basename "foo.js" (in commands, stacktraces, editor tabs, whereever) unambigously refer to library code and "foo.test.js" to test code would be helpful.

Earlier discussion here: #2128 (comment)

FYI: some very much incomplete "migrate from tape to tap" PoC work here: https://github.com/elastic/apm-agent-nodejs/tree/trentm/tap/