Fix test for nightly panic message change

A recent nightly changed the format of the panic message. This updates
the test that was matching against this so it doesn't match the text
that has changed.

Author: ehuss

tests/testsuite/test.rs

@@ -35,7 +35,7 @@ fn failing_tests() {
 ...
 test failing.md - Failing_Tests (line 3) ... FAILED
 ...
-thread 'main' panicked at failing.md:3:1:
+thread [..] panicked at failing.md:3:1:
 fail
 ...
 [TIMESTAMP] [INFO] (mdbook_driver::mdbook): Testing chapter 'Failing Include': "failing_include.md"
@@ -45,7 +45,7 @@ fail
 ...
 test failing_include.md - Failing_Include (line 3) ... FAILED
 ...
-thread 'main' panicked at failing_include.md:3:1:
+thread [..] panicked at failing_include.md:3:1:
 failing!
 ...
 [TIMESTAMP] [ERROR] (mdbook_core::utils): Error: One or more tests failed