rna-transcription: add test to avoid cheesing (#2151)

In the normal tests for this exercise, we compare two RNA instances. If
the user defines RNA as an empty struct, those instances will always
compare equal. This is prevented with the new test to make sure
different RNA don't compare equal.

Forum post:
https://forum.exercism.org/t/successful-null-solution-for-rna-transcription/63064

Author: senekor

exercises/practice/rna-transcription/.meta/test_template.tera

@@ -1,4 +1,21 @@
 use rna_transcription::{Dna, Rna};
+
+{#-
+    We usually do not add additional tests directly in the template.
+    In this case however, the structure of the tests is different from the
+    regular ones. Accommodating that in the template would be complicated
+    and unnecessary. We also want to test this early, so that users don't
+    make it through basically all tests without doing anything.
+#}
+
+#[test]
+#[ignore]
+fn different_rna_are_different() {
+    let a = Rna::new("G").unwrap();
+    let b = Rna::new("UGCACCAGAAUU").unwrap();
+    assert_ne!(a, b);
+}
+
 {% for test in cases %}
 #[test]
 #[ignore]

exercises/practice/rna-transcription/tests/rna_transcription.rs

@@ -1,6 +1,14 @@
 use rna_transcription::{Dna, Rna};
 
 #[test]
+fn different_rna_are_different() {
+    let a = Rna::new("G").unwrap();
+    let b = Rna::new("UGCACCAGAAUU").unwrap();
+    assert_ne!(a, b);
+}
+
+#[test]
+#[ignore]
 fn empty_rna_sequence() {
     let input = "";
     let output = Dna::new(input).unwrap().into_rna();