testsuite: cover exclusion with dynamic R

Problem: there is no test coverage for excluding ranks from
an instance that discovers R dyanmically.

Add test to t2312-resource-exclude.t.

t/t2312-resource-exclude.t

@@ -109,5 +109,14 @@ test_expect_success 'flux resource status shows zero nodes excluded' '
 test_expect_success 'unexclude event was posted' '
 	test $(has_resource_event unexclude | wc -l) -eq 1
 '
+# See flux-framework/flux-core#5337
+test_expect_success 'test instance can exclude ranks' '
+	cat >exclude.toml <<-EOT &&
+	[resource]
+	exclude = "1"
+	EOT
+	test $(flux start -s2 -o,--config-path=exclude.toml \
+	    flux resource status -s exclude -no {nnodes}) -eq 1
+'
 
 test_done