Add custom reporter to reduce the test output

jest.config.js

@@ -15,6 +15,7 @@ module.exports = {
     // Replaces the following formats with an empty module.
     '^.+\\.(scss|css|svg|woff|woff2|mp4|webm)$': '<rootDir>/tests/emptyModule',
   },
+  reporters: ['<rootDir>/tests/jest-reporters/fingers-crossed.js'],
   setupTestFrameworkScriptFile: '<rootDir>/tests/setup.js',
   testPathIgnorePatterns: [
     '<rootDir>/node_modules/',

tests/jest-reporters/fingers-crossed.js

@@ -0,0 +1,33 @@
+/* eslint import/no-extraneous-dependencies: 0 */
+const chalk = require('chalk');
+const { getConsoleOutput } = require('jest-util');
+const DefaultReporter = require('jest-cli/build/reporters/default_reporter')
+  .default;
+const getResultHeader = require('jest-cli/build/reporters/get_result_header')
+  .default;
+
+const TITLE_BULLET = chalk.bold('\u25cf ');
+
+// This Jest reporter does not output any console.log except when the tests are
+// failing, see: https://github.com/mozilla/addons-frontend/issues/2980.
+class FingersCrossedReporter extends DefaultReporter {
+  printTestFileHeader(testPath, config, result) {
+    this.log(getResultHeader(result, this._globalConfig, config));
+
+    const consoleBuffer = result.console;
+    const testFailed = result.numFailingTests > 0;
+
+    if (testFailed && consoleBuffer && consoleBuffer.length) {
+      // prettier-ignore
+      this.log(
+        `  ${TITLE_BULLET}Console\n\n${getConsoleOutput(
+          config.cwd,
+          !!this._globalConfig.verbose,
+          consoleBuffer
+        )}`
+      );
+    }
+  }
+}
+
+module.exports = FingersCrossedReporter;