Add test/unittests/README.md

For those new to the repository and unit testing, we add a README
that describes the most common tasks for unit testing.

R=dsv@chromium.org

Fixed: 1221492
Change-Id: Iae43609c7a02b09a8d5056fe280176a74bce904c
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2975284
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Commit-Queue: Danil Somsikov <dsv@chromium.org>
Auto-Submit: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Danil Somsikov <dsv@chromium.org>

test/unittests/README.md

@@ -0,0 +1,42 @@
+# Unit tests
+
+You can run the unit tests with `npm run auto-unittest`.
+Unit tests are written using [Mocha](https://mochajs.org/) and [Chai](https://www.chaijs.com/) and run with [Karma](https://karma-runner.github.io/latest/index.html) in a web browser.
+
+## Running a subset of unit tests
+If you want to run a specific (set of) unit test, you can use `it.only` or `describe.only` for those tests that you want to run.
+
+```ts
+describe.only('The test suite that you want to run', () => {
+  it('A test that would run', () => {});
+  it('Another test that would run', () => {});
+});
+```
+
+```ts
+describe('The test suite that you want to run', () => {
+  it.only('A test that would run', () => {});
+  it('A test that would not run', () => {});
+});
+```
+
+## Obtaining code coverage
+
+We can collect code coverage for the source code that is tested with unit tests.
+You can run `COVERAGE=1 npm run auto-unittest` to obtain code coverage for the whole `front_end` folder.
+However, there is some preprocessing overhead for collecting coverage.
+You can use `COVERAGE_FOLDERS` to only preprocess specific folders for code coverage:
+
+```bash
+COVERAGE_FOLDERS='front_end/ui/components' npm run auto-unittest
+COVERAGE_FOLDERS='front_end/{ui/components,core/common}' npm run auto-unittest
+```
+
+## Inspecting detailed errors
+
+By default, the Karma testing output is terse, to avoid console output cluttering.
+However, if you want to obtain detailed reporting of a failure you are investigating, you can use `--expanded-reporting`:
+
+```bash
+npm run auto-unittest -- --expanded-reporting
+```