test: fix flaky stack_submit_with_assignees

Replace cmpenv stderr golden file comparison with individual
stderr assertions. The exact comparison was flaky because
a "Failed to cache templates" warning could non-deterministically
appear in stderr when the template caching timeout fires.

Individual stderr assertions match the pattern used by
stack_submit.txt and are tolerant of extra log lines.

Author: ed-irl

testdata/script/stack_submit_with_assignees.txt

@@ -36,7 +36,9 @@ git config spice.submit.assignees spice
 # submit the entire stack from the middle.
 git checkout feature1
 gs stack submit --fill --assign alex --assign beatrice,cameron
-cmpenv stderr $WORK/golden/submit-log.txt
+stderr 'Created #1'
+stderr 'Created #2'
+stderr 'Created #3'
 
 shamhub dump changes
 cmpenvJSON stdout $WORK/golden/start.json
@@ -61,10 +63,6 @@ This is feature 2
 -- repo/feature3.txt --
 This is feature 3
 
--- golden/submit-log.txt --
-INF Created #1: $SHAMHUB_URL/alice/example/change/1
-INF Created #2: $SHAMHUB_URL/alice/example/change/2
-INF Created #3: $SHAMHUB_URL/alice/example/change/3
 -- golden/start.json --
 [
   {