[fix] Test configuration

[ckardaris]

• Collect tests in sub-directories.
• Fix loading of settings for tests.

Description

This PR is based on discussion threads in #4880.

The Settings.load() call is necessary because the tests would fail to run on my machine and I was getting the following error output.

$ ./make.js test
run: started
test-unit: started
Fail "UIComponent: focus the frame when showing"
Error: Getting the setting userDefinedLinkHintCss before settings have been loaded.
    at Object.get (file:///home/charalampos.kardaris/src/vimium/lib/settings.js:137:13)
    at Object.injectUserCss (file:///home/charalampos.kardaris/src/vimium/lib/dom_utils.js:566:34)
    at UIComponent.load (file:///home/charalampos.kardaris/src/vimium/content_scripts/ui_component.js:63:14)
    at Object.<anonymous> (file:///home/charalampos.kardaris/src/vimium/tests/unit_tests/ui_component_test.js:35:13)
    at Object.runTest (file:///home/charalampos.kardaris/src/vimium/tests/vendor/shoulda.js:253:29)
    at eventLoopTick (ext:core/01_core.js:187:7)
    at async Object.runContext (file:///home/charalampos.kardaris/src/vimium/tests/vendor/shoulda.js:214:9)
    at async Object.run (file:///home/charalampos.kardaris/src/vimium/tests/vendor/shoulda.js:187:7)
    at async runUnitTests (file:///home/charalampos.kardaris/src/vimium/make.js:283:10)
    at async Task.<anonymous> (file:///home/charalampos.kardaris/src/vimium/make.js:351:19)
---
TypeError: undefined is not iterable (cannot read property Symbol(Symbol.iterator))
    at Object.<anonymous> (file:///home/charalampos.kardaris/src/vimium/tests/unit_tests/ui_component_test.js:28:25)
    at Object.runTest (file:///home/charalampos.kardaris/src/vimium/tests/vendor/shoulda.js:261:38)
    at eventLoopTick (ext:core/01_core.js:187:7)
    at async Object.runContext (file:///home/charalampos.kardaris/src/vimium/tests/vendor/shoulda.js:214:9)
    at async Object.run (file:///home/charalampos.kardaris/src/vimium/tests/vendor/shoulda.js:187:7)
    at async runUnitTests (file:///home/charalampos.kardaris/src/vimium/make.js:283:10)
    at async Task.<anonymous> (file:///home/charalampos.kardaris/src/vimium/make.js:351:19)
    at async Promise.all (index 0)
    at async TaskRegistry.execute (https://deno.land/x/drake@v1.5.1/lib/tasks.ts:446:5)
    at async TaskRegistry.run (https://deno.land/x/drake@v1.5.1/lib/tasks.ts:379:9)
Fail (1/244)
error: test-unit failed