Adds benchmark support to gotest.

Adds benchmark support to gotest.

No benchmarks are run unless the --benchmarks=<regexp> flag
is specified on the gotest command line.  This change includes
sample benchmarks for regexp.

% gotest --benchmarks=.*
(standard test output redacted)
testing.BenchmarkSimpleMatch	200000	      7799 ns/op
testing.BenchmarkUngroupedMatch	20000	     76898 ns/op
testing.BenchmarkGroupedMatch	50000	     38148 ns/op

R=r, rsc
https://golang.org/cl/154173