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fix: Improve web-ext start-up time by lazy loading some modules (#1302)

* fix: Lazy-load command modules to speed up start-up

Simple commands like `web-ext --help` now require less than a second
instead of 4 seconds to complete.

* test: refactor failing test and improve coverage

A previous commit changed `src/cmd/index.js` to not directly export the
implementations, but to lazy-load those modules.
So the test that did a simple comparison started failing.

To fix this test and maintain test coverage, change the test to
temporarily intercept invocations of the original cmd module,
for every command that is exported in `src/cmd/index.js`.

* fix: Lazy-load runner modules to speed up "web-ext run"

Boosts start-up performance by about 400ms.
Benchmarked as follows:

1. Create extension directory (manifest.json)
2. Start web-ext:

        time path/to/web-ext/bin/web-ext run -f/dev/null >/dev/null

   (The first `/dev/null` is to not let Firefox affect the timings,
    the program will crash with EACCESS as soon as it tries to
    spawn `/dev/null` as a program).
3. Repeat 10 times and measure the real time.

* refactor: Move runner module loading from cmd/run.js to extension-runners/index.js

Also, remove the optional FirefoxDesktopExtensionRunner and
FirefoxAndroidExtensionRunner parameters from cmd/run, since these were
only used in tests.
Now tests/unit/test-cmd/test.run.js detects runner creations by
stubbing the actual implementations instead of injecting an object.

* refactor: move prepareRun test helper outside describe

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