Move basic perf-test output from perftest.log to perftest.sum.

Move basic perf-test output from perftest.log to perftest.sum.

This patch does two things.
1) Add support for multiple data points.
2) Move the "report" output from perftest.log to perftest.sum.

I want to record the raw data somewhere, and a bit of statistical analysis
(standard deviation left for another day), but I also don't want
it to clutter up the basic report.
This patch takes a cue from gdb.{sum,log} and does the same thing
with perftest.{sum,log}.
Ultimately, we'll probably want to emit raw data to csv files or some
such and then do post-processing passes on that.

gdb/testsuite/ChangeLog:

	* lib/perftest/reporter.py (SUM_FILE_NAME): New global.
	(LOG_FILE_NAME): New global.
	(TextReporter.__init__): Initialize self.txt_sum.
	(TextReporter.report): Add support for multiple data-points.
	Move report to perftest.sum, put raw data in perftest.log.
	(TextReporter.start): Open sum and log files.
	(TextReporter.end): Close sum and log files.
	* lib/perftest/testresult.py (SingleStatisticTestResult.record): Handle
	multiple data-points.