xunit output: add backtraces; limit attribute

When pexpect errors occurred, the <error>/<failure> element's
message attribute could get too long and contain
invalid characters for xml attributes, even when quoted.
Particularly for pexpect failures.

Now <error> and <failure> entries truncate the message
attribute to contain the first line of the message.

<error> and <failure> blocks now contain both the
complete message and the backtrace (finally!) in the
text body of the error/failure element.

llvm-svn: 247973