move testutil.go to testutil_test.go to avoid testing module setting fla...

...gs.

Using this module causes the side effect of setting up testing flags. See example program: https://gist.github.com/daniel-garcia/c952045307249e69e1f9

The resulting binary will have all testing flags appended:

Usage of ./test1:
  -logging="debug": Which log level: [debug,info,warn,error,fatal]
  -test.bench="": regular expression to select benchmarks to run
  -test.benchmem=false: print memory allocations for benchmarks
  -test.benchtime=1s: approximate run time for each benchmark
  -test.blockprofile="": write a goroutine blocking profile to the named file after execution
  -test.blockprofilerate=1: if >= 0, calls runtime.SetBlockProfileRate()
  -test.cpu="": comma-separated list of number of CPUs to use for each test
  -test.cpuprofile="": write a cpu profile to the named file during execution
  -test.memprofile="": write a memory profile to the named file after execution
  -test.memprofilerate=0: if >=0, sets runtime.MemProfileRate
  -test.parallel=1: maximum test parallelism
  -test.run="": regular expression to select tests and examples to run
  -test.short=false: run smaller test suite to save time
  -test.timeout=0: if positive, sets an aggregate time limit for all tests
  -test.v=false: verbose: print additional output