test for speed of Bazel's sandboxing code

BUILD

inputs = ['input_%s' % i for i in range(3000)]
genrules = ['genrule_%s' % i for i in range(100)]

genrule(
  name = 'gen_inputs',
  outs = inputs,
  cmd = 'touch ' + ' '.join(['$(location %s)' % i for i in inputs]),
)

[genrule(
  name = 'gen_%s' % name,
  srcs = inputs,
  outs = [name],
  cmd = 'touch $@',
) for name in genrules]

genrule(
  name = 'gen_main',
  srcs = genrules,
  outs = ['main'],
  cmd = 'touch $@',
)