Enable single thread mode for runner

Some runners for tools that can not be run in parallel need a way
to specify that they should not be executed in parallel. This commits
adds this functionality.

Closes jestjs#5706

CHANGELOG.md

@@ -14,6 +14,8 @@
   promises ([#5670](https://github.com/facebook/jest/pull/5670))
 * `[expect]` Add isError to utils
   ([#5670](https://github.com/facebook/jest/pull/5670))
+* `[jest-cli]` Add `serial` property that runners can expose to specify that
+  they can not run in parallel (#5706)
 
 ### Fixes
 

docs/Configuration.md

@@ -703,6 +703,10 @@ async runTests(
 ): Promise<void>
 ```
 
+If you need to restrict your test-runner to only run in serial rather then being
+exectued in paralell your class should have the static attribute `serial` to be
+set as `true`.
+
 ### `setupFiles` [array]
 
 Default: `[]`

packages/jest-cli/src/__tests__/test_scheduler.test.js

@@ -12,6 +12,11 @@ import TestScheduler from '../test_scheduler';
 import SummaryReporter from '../reporters/summary_reporter';
 
 jest.mock('../reporters/default_reporter');
+const mockSerialRunner = {
+  serial: true,
+  runTests: jest.fn()
+};
+jest.mock('jest-runner-serial', () => jest.fn(() => mockSerialRunner));
 
 test('.addReporter() .removeReporter()', () => {
   const scheduler = new TestScheduler({}, {});
@@ -21,3 +26,23 @@ test('.addReporter() .removeReporter()', () => {
   scheduler.removeReporter(SummaryReporter);
   expect(scheduler._dispatcher._reporters).not.toContain(reporter);
 });
+
+test('serial option in runner call runTests with serial', async () => {
+  const scheduler = new TestScheduler({}, {});
+  const tests = [{
+    context: {
+      config: {
+        runner: 'jest-runner-serial'
+      },
+      hasteFS: HasteFS,
+      moduleMap: ModuleMap,
+      resolver: HasteResolver,
+    },
+    path: './test/path.js',
+  }]
+  await scheduler.scheduleTests(tests);
+  const jestRunnerSerial = require('jest-runner-serial');
+
+  expect(jestRunnerSerial).toHaveBeenCalled();
+  mockSerialRunner.runTests.toHaveBeenCalled();
+});

packages/jest-cli/src/test_scheduler.js

@@ -192,7 +192,7 @@ export default class TestScheduler {
             onResult,
             onFailure,
             {
-              serial: runInBand,
+              serial: runInBand || testRunners[runner].serial,
             },
           );
         }