tests: fold more small tests into main.rs

Put the insta testdata expected value inline in the file, so that it's
not accidentally deleted when Insta is asked to delete unreferenced
snapshots in the main tree.

Author: sourcefrog

testdata/insta/tests/insta.rs

@@ -3,5 +3,5 @@ use cargo_mutants_testdata_insta::say_hello;
 #[test]
 fn say_hello_vs_insta_snapshot() {
     let name = "Robin";
-    insta::assert_snapshot!(say_hello(name));
+    insta::assert_snapshot!(say_hello(name), @r###"Hello, Robin!"###);
 }