test: fix box-tap/gh-5602-environment-vars-cfg

Fix the `box-tap/gh-5602-environment-vars-cfg.test.lua` diff test after
disabling stdout bufferization in test-run.

NO_DOC=test
NO_TEST=test
NO_CHANGELOG=test

(cherry picked from commit d2d06824)

test/box-tap/gh-5602-environment-vars-cfg.result

 TAP version 13
+1..1
+TAP version 13
 1..6
 ok - box.cfg is successful
 ok - listen
@@ -28,6 +30,4 @@ TAP version 13
 1..2
 ok - box.cfg is not successful
 ok - bad strip_core value
-TAP version 13
-1..1
 ok - exit status list