Support jest coverage output

[gregmagolan]

See aspect-build/rules_js#337 for more context

[supercairos]

If it helps anyone,
I've managed to tricks Bazel into reading the outputed jest lcov using this jest config:

const path = require('path');

module.exports = {
  testEnvironment: 'node',
  reporters: ['default', ['jest-junit', { outputFile: process.env.XML_OUTPUT_FILE }]],
  testMatch: ['**/*.{test,spec}.js'],
  setupFiles: ['./test/.jest/set_env_vars.js'],
  collectCoverage: true,
  coverageDirectory: path.dirname(process.env.COVERAGE_OUTPUT_FILE),
  coverageReporters: ['text', ['lcovonly', { file: path.basename(process.env.COVERAGE_OUTPUT_FILE) }]],
};

[gregmagolan]

This should be easier to add now that jest_test generates a jest config file (added in 1141e1b) that we can add

  collectCoverage: true,
  coverageDirectory: path.dirname(process.env.COVERAGE_OUTPUT_FILE),
  coverageReporters: ['text', ['lcovonly', { file: path.basename(process.env.COVERAGE_OUTPUT_FILE) }]],

to when coverage is enabled