message: Allow tests to be ignored
diff: use libtest_mimic::{Outcome, Test};
  use std::fmt::Write;
  +use std::fs;
  use std::path::{Path, PathBuf};
  use std::process::Command;
  use walkdir::WalkDir;
  @@ -40,6 +41,11 @@ pub fn run_test(
  }
  fn run_test_impl(pikelet_exe: &str, test: &Test<TestData>) -> Outcome {
  + let input_source = fs::read_to_string(&test.data.input_file).unwrap();
  + if input_source.starts_with("--! IGNORE") {
  + return Outcome::Ignored;
  + }
  +
  let output = Command::new(pikelet_exe)
  .arg("check")
  .arg("--validate-core")
  