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/vim9compile.c b/src/vim9compile.c
index 64ed038..158770c 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -666,7 +666,7 @@
 {
     isn_T	*isn;
 
-    if ((isn = generate_instr_type(cctx, ISN_PUSHCHANNEL, &t_channel)) == NULL)
+    if ((isn = generate_instr_type(cctx, ISN_PUSHJOB, &t_channel)) == NULL)
 	return FAIL;
     isn->isn_arg.job = job;