Fix nil error with KNOWNBUGS.rb

Previously, `make test-knownbugs` crashed with `NoMethodError` due to
the failed regex match if there is a test case in KNOWNBUGS.rb.

The note about 1.8 compatibility is probably bogus as we require a way
more recent BASERUBY now.

bootstraptest/runner.rb

@@ -6,7 +6,6 @@
 # Never use optparse in this file.
 # Never use test/unit in this file.
 # Never use Ruby extensions in this file.
-# Maintain Ruby 1.8 compatibility for now
 
 $start_time = Time.now
 
@@ -428,7 +427,7 @@ def add as
   def initialize(*args)
     super
     self.class.add self
-    @category = self.path.match(/test_(.+)\.rb/)[1]
+    @category = self.path[/\Atest_(.+)\.rb\z/, 1]
   end
 
   def call