Ensure watcher tests don't block during shutdown

These test failures from #93 inspired this change:
https://travis-ci.org/bitly/google_auth_proxy/jobs/62474406
https://travis-ci.org/bitly/google_auth_proxy/jobs/62474407

Both tests exhibited this pattern:
2015/05/13 22:10:54 validating: is [email protected] valid? false
2015/05/13 22:10:54 watching interrupted on event: "/tmp/test_auth_emails_300880185": CHMOD
2015/05/13 22:10:54 watching resumed for /tmp/test_auth_emails_300880185
2015/05/13 22:10:54 reloading after event: "/tmp/test_auth_emails_300880185": CHMOD
panic: test timed out after 1m0s

[snip]

goroutine 175 [chan send]:
github.com/bitly/google_auth_proxy.(*ValidatorTest).TearDown(0xc2080bc330)
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_test.go:27 +0x43
github.com/bitly/google_auth_proxy.TestValidatorOverwriteEmailListViaRenameAndReplace(0xc2080f2480)
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_watcher_test.go:103 +0x3b9

[snip]

goroutine 177 [chan send]:
github.com/bitly/google_auth_proxy.func·017()
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_test.go:34 +0x41

I realized that the spurious CHMOD events were causing calls to
`func() { updated <- true }` (from validator_test.go:34), which caused
the goroutine running the watcher to block. At the same time,
ValidatorTest.TearDown was blocked by trying to send into the `done` channel.
The solution was to create a flag that ensured only one value was ever sent
into the update channel.

Author: Mike Bland

validator_test.go

@@ -10,6 +10,7 @@ import (
 type ValidatorTest struct {
 	auth_email_file *os.File
 	done            chan bool
+	update_seen     bool
 }
 
 func NewValidatorTest(t *testing.T) *ValidatorTest {
@@ -31,7 +32,12 @@ func (vt *ValidatorTest) TearDown() {
 func (vt *ValidatorTest) NewValidator(domains []string,
 	updated chan<- bool) func(string) bool {
 	return newValidatorImpl(domains, vt.auth_email_file.Name(),
-		vt.done, func() { updated <- true })
+		vt.done, func() {
+			if vt.update_seen == false {
+				updated <- true
+				vt.update_seen = true
+			}
+		})
 }
 
 // This will close vt.auth_email_file.