patch 8.2.1839: Vim9: memory leaks reported in assign test

Problem:    Vim9: memory leaks reported in assign test.
Solution:   Move the failing job_start() call to separate test files, it
            causes false leak reports.

src/testdir/test_vim9_fails.vim

@@ -0,0 +1,10 @@
+" Test for Vim9 script with failures, causing memory leaks to be reported.
+" The leaks happen after a fork() and can be ignored.
+
+def Test_assignment()
+  if has('channel')
+    var chan1: channel
+    var job1: job
+    var job2: job = job_start('willfail')
+  endif
+enddef