rename testutils.go to testutils_test.go

testutils.go imports "testing" but is included in normal, non-test
builds. This results in any package that imports "nosurf" and also
"flags" having its command line flags polluted by testing flags, such as
in the included example.

    Usage of ./translator:
      -c="": Path to the config file
      -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.coverprofile="": write a coverage profile to the named file after execution
      -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.outputdir="": directory in which to write profiles
      -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

It is a bit weird to have both utils_test.go and testutils_test.go, but
0d01914c1a4e23eb4f3e9afe61ce12e4970d4722 renamed test_utils.go to
testutils.go to "differentiate it from *_test.go" files. As stated
previously, we can't make the file not part of the *_test.go set, but we
can still differentiate it from the utils_test.go file.