Exclude the source dir for the C-ext backtraces test

* Now that it differs from the Ruby home.

Author: eregon

test/truffle/cexts/backtraces/bin/backtraces

@@ -1,12 +1,12 @@
 #!/usr/bin/env ruby
-
 require 'backtraces'
 
 TEST_DIR = File.expand_path("../..", __FILE__)
+SRC_DIR = File.expand_path("../../../../../..", __FILE__)
 
 def puts_backtrace(backtrace)
-  puts backtrace.gsub(TEST_DIR, '')
-                .gsub(Truffle::Boot.ruby_home, '')
+  puts backtrace.gsub(Truffle::Boot.ruby_home, '')
+                .gsub(TEST_DIR, '').gsub(SRC_DIR, '')
                 .gsub(/\/(cext\.rb|cext_ruby\.rb|ruby\.c|qsort\.c):(\d+:)?/, '/\1:n:')
                 .gsub(/@\h+/, '@HEXA')
                 .gsub(/\{id: \d+ name: [^}]+}/, 'BLOCKINFO')