Add spec for Thread::Backtrace::Location#absolute_path with a relative main script

(cherry picked from commit 767f295)

Author: eregon

spec/ruby/core/thread/backtrace/location/absolute_path_spec.rb

@@ -10,6 +10,13 @@
     @frame.absolute_path.should == File.realpath(__FILE__)
   end
 
+  it 'returns an absolute path when using a relative main script path' do
+    script = fixture(__FILE__, 'absolute_path_main.rb')
+    Dir.chdir(File.dirname(script)) do
+      ruby_exe('absolute_path_main.rb').should == "absolute_path_main.rb\n#{script}\n"
+    end
+  end
+
   context "when used in eval with a given filename" do
     it "returns filename" do
       code = "caller_locations(0)[0].absolute_path"

spec/ruby/core/thread/backtrace/location/fixtures/absolute_path_main.rb

@@ -0,0 +1,2 @@
+puts __FILE__
+puts caller_locations(0)[0].absolute_path

spec/tags/core/thread/backtrace/location/absolute_path_tags.txt

@@ -0,0 +1 @@
+slow:Thread::Backtrace::Location#absolute_path returns an absolute path when using a relative main script path