patch 8.2.0343: Vim9: using wrong instruction, limited test coverage

Problem:    Vim9: using wrong instruction, limited test coverage.
Solution:   Use ISN_PUSHJOB.  Add a few more tests.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index c1865e3..c01ab83 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -56,6 +56,7 @@
   if has('channel')
     let chan1: channel
     let job1: job
+    let job2: job = job_start('willfail')
   endif
   if has('float')
     let float1: float = 3.4