doc(testing/components): Clarify Testing a Custom Element

[baerrach]

Removed inform box specific to Karma as there is no documentation at all about
how to set Karma up.

Fixes #441

[EisenbergEffect]

Was this the final outcome of the conversation or were we waiting on @3cp to respond to #441 ?

[zewa666]

@EisenbergEffect I'd like @3cp to chime into this discussion as he has a better overview of how Karma, Jest and other fit into the CLI.

[3cp]

With cli, it definitely has no need of src/ prefix, for all karma/jest/cli-bundler/webpack. The doc is outdated I guess.

But with webpack+karma, you do need .withResources(PLATFORM.moduleName('some-comp')) wrapper for webpack to trace it.

The jest has config inside app package.json to skip the src/ prefix.

"modulePaths": [
      "<rootDir>/src",
      "<rootDir>/node_modules"
    ],