gtest: modified Xcode integration so gtest assert/expect failure now shows...

gtest: modified Xcode integration so gtest assert/expect failure now shows error content in the error marker at the line of failure.

This change modifies the python test runner to combine lines starting with a
failure report including up to 3 more lines, terminated by the next
"[  FAILED  ]" line.  These are all emitted on the same line as the file:line
indication, which allows Xcode's failure marker code to pick it up and display
it along with the error badge in the Xcode editor window.  Makes for a nice
gtest development experience.

llvm-svn: 218518