Fix callbacks_test path separator on Windows.

In cl/234179031, we improved this test to use `os.path.join` rather than
a hard-coded `+ "/tb"` in a few places, but apparently missed some.

Tested:
After 1036s/tb/something_else/, tests still pass on my local gLinux
desktop. Before this change, tests would fail after that substitution,
albeit not for the same reason as the traceback in b/125762969: we get a
normal `assertItemsEqual` failure rather than an ENOENT. Running Kokoro
Windows presubmits per go/tf-testing passes.

RELNOTES: n/a
PiperOrigin-RevId: 235232824