test: allow the allow-path modules' #[path] test side files in the key guard

0b146ef7 moved decrypt.rs and sector/decrypting.rs tests into
decrypt_tests.rs, decrypt_spec_guards_tests.rs and sector/decrypting_tests.rs;
they are the same modules' tests, so they share the allow-path.

tests/legacy_key_api_guard.rs

@@ -31,8 +31,16 @@ fn allowed(rel: &str) -> bool {
         || rel.starts_with("src/aacs/")
         || rel == "src/decrypt.rs"
         || rel == "src/sector/decrypting.rs"
+        || TEST_SIDE_FILES.contains(&rel)
 }
 
+/// The `#[cfg(test)]` side files of the allow-path modules above (`#[path]` tests).
+const TEST_SIDE_FILES: [&str; 3] = [
+    "src/decrypt_tests.rs",
+    "src/decrypt_spec_guards_tests.rs",
+    "src/sector/decrypting_tests.rs",
+];
+
 /// `src` with every comment blanked (newlines kept), and a copy with string and char
 /// literal contents blanked too; both keep `src`'s char positions.
 fn lex(src: &str) -> (Vec<char>, Vec<char>) {
@@ -250,6 +258,8 @@ fn the_structural_guard_skips_comments_and_keeps_its_allow_path() {
         "src/aacs/content.rs",
         "src/decrypt.rs",
         "src/sector/decrypting.rs",
+        "src/decrypt_tests.rs",
+        "src/sector/decrypting_tests.rs",
     ] {
         assert!(hits(rel, code).is_empty(), "{rel} is on the allow-path");
     }