Should a test file be placed in the same folder as the source file?

Is it easier to place test files right next to the source files they test (in the same src directory), or should a separate tests directory with a mirror hierarchy be created?

Having them in the same folder seems to make life easier when it comes to maintenance, but also crowds the source directories.

Option 1: Directory structure when having separate folders for source and tests:

- src
+-- item.ts
`-- util
    +-- helper.ts

- test
+-- item.test.ts
`-- util
    `-- helper.test.ts

Option 2: Directory structure when having both types of files in the same directory:

- src
+-- item.ts
+-- item.test.ts
`-- util
    +-- helper.ts
    `-- helper.test.ts

I used to always go with option 1 until trying out angular-cli and having it generate code files alongside the test files as in option 2, which made me rethink the whole thing.