fix: port collisions in createTestContext (prisma-labs#1326)

- during testing, get-port often re-uses the same port, due to the delay between when a free-port is assigned to a test, and when the port is actually used. Simply supplying an initial random port value to get-port dramatically reduces the chance of a port being used multiple times in a test run.
- testing on CI environments should still utilize Jest’s `—runInBand` flag

src/testing/testing.ts

@@ -44,7 +44,8 @@ export async function createTestContext(opts?: CreateTestContextOptions): Promis
     await Layout.create({ entrypointPath: opts?.entrypointPath, projectRoot: opts?.projectRoot })
   )
   const pluginManifests = await PluginWorktime.getUsedPlugins(layout)
-  const randomPort = await getPort({ port: getPort.makeRange(4000, 6000) })
+  const randomStartPort = Math.floor(Math.random() * (6000 - 4000)) + 4000
+  const randomPort = await getPort({ port: getPort.makeRange(randomStartPort, 6000) })
   const privateApp = app as PrivateApp
 
   const forcedServerSettings = {