test(cache): ignore coverage for corrupted cache check (#401)

- similar to the other safety checks in `clean`, this won't be hit during normal usage

src/tscache.ts

@@ -265,7 +265,7 @@ export class TsCache
 				cache.write(hash, data);
 				return data;
 			}
-			else
+			else /* istanbul ignore next -- should only happen when corrupted cache */
 				this.context.warn(yellow("    cache broken, discarding"));
 		}