patch 8.0.1771: in tests, when WaitFor() fails it doesn't say why

Problem:    In tests, when WaitFor() fails it doesn't say why. (James McCoy)
Solution:   Add WaitForAssert(), which produces an assert error when it fails.
diff --git a/src/testdir/test_job_fails.vim b/src/testdir/test_job_fails.vim
index 22637c0..affcb7d 100644
--- a/src/testdir/test_job_fails.vim
+++ b/src/testdir/test_job_fails.vim
@@ -8,9 +8,9 @@
   if has('job')
     let job = job_start('axdfxsdf')
     if has('unix')
-      call WaitFor({-> job_status(job) == "dead"})
+      call WaitForAssert({-> assert_equal("dead", job_status(job))})
     else
-      call WaitFor({-> job_status(job) == "fail"})
+      call WaitForAssert({-> assert_equal("fail", job_status(job))})
     endif
   endif
 endfunc