Do not attempt to write snapshot file in runner

The runner frequently has a very long snapshot filename, since it
includes all the CLI arguments, and people may run tap like
`tap lib/**/*.test.js` and pull in a lot of files for a large program.

Since the runner never has a snapshot to save, setting it in
writeSnapshot mode means that it will try to rm the file, resulting in
an ENAMETOOLONG error.

Close #567

bin/run.js

@@ -416,6 +416,7 @@ const stdinOnly = options => {
   // to the reporter, so we don't get '/dev/stdin' in the suite list.
   // We have to pause() before piping to switch streams2 into old-mode
   const tap = require('../lib/tap.js')
+  tap.writeSnapshot = false
   tap.stdinOnly()
   makeReporter(tap, options)
 
@@ -665,6 +666,7 @@ const runTests = options => {
   // At this point, we know we need to use the tap root,
   // because there are 1 or more files to spawn.
   const tap = require('../lib/tap.js')
+  tap.writeSnapshot = false
   if (options.comments) {
     const onComment = c => {
       if (!c.match(/^# (time=[0-9\.]+m?s|Subtest(: .+)?)\n$/))