testsuite: update shell taskmap tests

Problem: No tests in the testsuite check that a taskmap which
changes task counts amongst nodes works with the job shell. Also,
one expected failure now succeeds because this type of taskmap is
now allowed.

Remove the failing test.

Add a test that assigns 3 tasks to rank 0 and one task on other
nodes.

Author: grondo

t/t2616-job-shell-taskmap.t

@@ -136,6 +136,15 @@ test_expect_success 'taskmap=manual works' '
 		| jq -e ".context.taskmap.map == [[1,3,4,1],[0,1,4,1]]" &&
 	test_check_taskmap $id
 '
+test_expect_success 'taskmap=manual can redistribute tasks amongst nodes' '
+	# put 2 extra tasks on rank 0
+	id=$(flux submit \
+	     --taskmap=manual:"[[0,1,3,1],[1,3,1,1]]" \
+	     -N4 -n6 ./map.sh) &&
+	flux job wait-event -p exec -f json $id shell.start \
+		| jq -e ".context.taskmap.map == [[0,1,3,1],[1,3,1,1]]" &&
+	test_check_taskmap $id
+'
 test_expect_success 'taskmap=manual with unknown taskmap fails' '
 	test_must_fail_or_be_terminated \
 		flux run --taskmap="manual:[]" true
@@ -149,8 +158,7 @@ test_expect_success 'invalid manual taskmaps fail predictably' '
 		-N4 --tasks-per-node=4 true ::: \
 		"{}" \
 		"[[1,3,4,1]]" \
-		"[[1,3,4,1],[0,1,3,1]]" \
-		"[[3,1,1,1],[0,3,5,1]]") &&
+		"[[1,3,4,1],[0,1,3,1]]") &&
 	for id in $ids; do
 		test_must_fail_or_be_terminated flux job attach $id
 	done