Expand docs about testing (#1251)

## Short description of the changes
Clarify the use of `jest` and add some notes about working with snapshot
tests. Also remove references to `.travis.yml`.

Signed-off-by: Máté Szabó <[email protected]>

BUILD.md

@@ -57,6 +57,16 @@ Runs after the top-level `yarn install`. This ensures `./packages/plexus` builds
 
 `tsc-lint-debug` is for diagnosing problems with linking, resolving files, or aliases in TypeScript code. It lists the files involved in the compilation.
 
+### `test`
+`test` runs tests for all packages.
+
+Note that `./packages/plexus` does not yet have any tests, as tracked in issue [#340](https://github.com/jaegertracing/jaeger-ui/issues/340).
+
+`./packages/jaeger-ui` uses [Jest](https://jestjs.io/) for testing. It can be useful to directly run tests for that package by running `yarn test` from its directory, rather than the repository root.
+
+Tests for React components in `./packages/jaeger-ui` make extensive use of Jest's [snapshot testing](https://jestjs.io/docs/28.x/snapshot-testing) functionality. These snapshots can be regenerated by running `yarn test -u` from the package directory to regenerate all snapshots,
+or `yarn test -u --testNamePattern <test name>` to regenerate snapshots for a subset of tests only.
+
 ### `husky` . `hooks` . `pre-commit`
 
 Runs the `lint` and `test` scripts.
@@ -67,9 +77,11 @@ Pretty basic.
 
 Note: This configuration is extended by `./packages/plexus/.eslintrc.js`.
 
-## `.travis.yml`
+## `.github/workflows`
+Holds GitHub Actions workflows used in CI and in release.
 
-Currently `./packages/plexus` doesn't have any tests... But, when it does, `.travis.yml` needs to be updated to send coverage info for all `./packages/*` to codecov.io. ([Ticket](https://github.com/jaegertracing/jaeger-ui/issues/340))
+CodeCov is integrated into the unit tests workflow to report coverage data from `./packages/jaeger-ui`.
+When unit tests are added to Plexus, this integration will need to be updated to gather coverage data for Plexus as well.
 
 [`yarn install --frozen-lockfile`](https://yarnpkg.com/lang/en/docs/cli/install/#toc-yarn-install-frozen-lockfile) ensures installs in CI fail if they would typically mutate the lockfile.