Move the filetest harness into its own crate.

This allows us to run the tests via a library call rather than just
as a command execution. And, it's a step toward a broader goal, which
is to keep the code in the top-level src directory minimal, with
important functionality exposed as crates.

lib/filetests/src/match_directive.rs

@@ -0,0 +1,27 @@
+/// Look for a directive in a comment string.
+/// The directive is of the form "foo:" and should follow the leading `;` in the comment:
+///
+/// ; dominates: ebb3 ebb4
+///
+/// Return the comment text following the directive.
+pub fn match_directive<'a>(comment: &'a str, directive: &str) -> Option<&'a str> {
+    assert!(
+        directive.ends_with(':'),
+        "Directive must include trailing colon"
+    );
+    let text = comment.trim_left_matches(';').trim_left();
+    if text.starts_with(directive) {
+        Some(text[directive.len()..].trim())
+    } else {
+        None
+    }
+}
+
+#[test]
+fn test_match_directive() {
+    assert_eq!(match_directive("; foo: bar  ", "foo:"), Some("bar"));
+    assert_eq!(match_directive(" foo:bar", "foo:"), Some("bar"));
+    assert_eq!(match_directive("foo:bar", "foo:"), Some("bar"));
+    assert_eq!(match_directive(";x foo: bar", "foo:"), None);
+    assert_eq!(match_directive(";;; foo: bar", "foo:"), Some("bar"));
+}