gdb/testsuite: Update worker thread show assertion

Commit 33ae4543 updated the text reported by GDB when showing the
number of worker threads. However, it neglected to update the assertions
using this text, which caused index-file.exp to fail. This commit
corrects this omission.

Tested index-file.exp is fixed on my local machine.

Approved-By: Tom Tromey <tom@tromey.com>