add hint about test.only

Author: ghuntley

components/server/src/bitbucket/README.md

@@ -8,3 +8,20 @@ export GITPOD_TEST_TOKEN_BITBUCKET='{ value: "$token", scopes: [] }'
 
 Replace `$token` with the integration test token.
 
+## Running a single test
+
+Use `test.only` thus:
+
+```js
+test('Your cool test', function (done) {
+    //...
+}
+```
+
+becomes
+
+```js
+test.only('Your cool test', function (done) {
+    //...
+}
+```