test: deal with RPC call causing Nvim to exit later

test/functional/vimscript/let_spec.lua

@@ -7,6 +7,7 @@ local eval = helpers.eval
 local meths = helpers.meths
 local exec = helpers.exec
 local exec_capture = helpers.exec_capture
+local expect_exit = helpers.expect_exit
 local source = helpers.source
 local testprg = helpers.testprg
 
@@ -28,7 +29,7 @@ describe(':let', function()
   it(":unlet self-referencing node in a List graph #6070", function()
     -- :unlet-ing a self-referencing List must not allow GC on indirectly
     -- referenced in-scope Lists. Before #6070 this caused use-after-free.
-    source([=[
+    expect_exit(100, source, [=[
       let [l1, l2] = [[], []]
       echo 'l1:' . id(l1)
       echo 'l2:' . id(l2)