terraform: run Gosched in test so other goroutines run

Starting with Go 1.8 betas, we've periodically received SIGQUITs on our
tests in Travis. The stack trace looks like this:

https://gist.github.com/mitchellh/abf09b0980f8ea01269f8d9d6133884d

The tests are timing out! This is a test that hasn't been touched really
in a very long time and has always passed. I've **reproduced this
locally** by setting `GOMAXPROCS=1` and running the test. By yielding
the scheduler in the hot loop, it now passes almost instantly every
time.

Perhaps the test can be written in a different way, but this gets tests
passing and I think will fix our periodic errors.

terraform/context_apply_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"reflect"
+	"runtime"
 	"sort"
 	"strings"
 	"sync"
@@ -1812,7 +1813,9 @@ func TestContext2Apply_cancelBlock(t *testing.T) {
 		close(applyCh)
 
 		for !ctx.sh.Stopped() {
-			// Wait for stop to be called
+			// Wait for stop to be called. We call Gosched here so that
+			// the other goroutines can always be scheduled to set Stopped.
+			runtime.Gosched()
 		}
 
 		// Sleep