Add watch option for test and bench subcommands

[Thomas Albertini]

Add watch option for deno test and deno bench.
I was trying this configuration and they don't work:

test: {
  desc: "Run test",
  cmd: "deno test ./tests/test.js",
  watch: true,
}, 

also:

test: {
  desc: "Run test",
  cmd: "deno test ./tests/test.js",
  watch: "true",
},

Also tried with yml configuration and/or passing the absolute path to the scripts. Am I missing something or are they absent? In latter case they would be a great addition