gdb/testsuite: Update worker thread show assertion

Commit 33ae45434d0 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>
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index d0990dc..7e478ab 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -10031,7 +10031,7 @@
 proc gdb_get_worker_threads { {testname ""} } {
     set worker_threads "UNKNOWN"
     gdb_test_multiple "maintenance show worker-threads" $testname {
-	-wrap -re "^The number of worker threads GDB can use is unlimited \\(currently ($::decimal)\\)\\." {
+	-wrap -re "^The number of worker threads GDB can use is the default \\(currently ($::decimal)\\)\\." {
 	    set worker_threads $expect_out(1,string)
 	}
 	-wrap -re "^The number of worker threads GDB can use is ($::decimal)\\." {