Add Tokenizer fuzz test

To test this, I reverted `aed02e737f7850026d736f46683d254b64a0c1af` and the fuzz test found
the crash within approximately 0.2 seconds.

src/aro/Tokenizer.zig

@@ -2150,6 +2150,28 @@ test "C23 keywords" {
     }, .c23);
 }
 
+test "Tokenizer fuzz test" {
+    var comp = Compilation.init(std.testing.allocator);
+    defer comp.deinit();
+
+    const input_bytes = std.testing.fuzzInput(.{});
+    if (input_bytes.len == 0) return;
+
+    const source = try comp.addSourceFromBuffer("fuzz.c", input_bytes);
+
+    var tokenizer: Tokenizer = .{
+        .buf = source.buf,
+        .source = source.id,
+        .langopts = comp.langopts,
+    };
+    while (true) {
+        const prev_index = tokenizer.index;
+        const tok = tokenizer.next();
+        if (tok.id == .eof) break;
+        try std.testing.expect(prev_index < tokenizer.index); // ensure that the tokenizer always makes progress
+    }
+}
+
 fn expectTokensExtra(contents: []const u8, expected_tokens: []const Token.Id, standard: ?LangOpts.Standard) !void {
     var comp = Compilation.init(std.testing.allocator);
     defer comp.deinit();

test/README.MD

@@ -84,7 +84,7 @@ If a test case is currently broken, it should be commented and a `TESTS_SKIPPED`
 // <something broken here>
 ```
 ---
-## Running fuzz tests
+## Running fuzz tests using AFLplusplus
 Fuzz testing requires [AFLplusplus](https://github.com/AFLplusplus/AFLplusplus). Run `zig build fuzz` to build the fuzz target,
 then `afl-fuzz -i test/cases -o test/fuzz-output -- ./zig-out/bin/arofuzz`
 
@@ -109,3 +109,8 @@ docker run --rm -it -v $PWD:/arocc -w /arocc --mount type=tmpfs,destination=/ram
 zig build fuzz  # This might take a while
 afl-fuzz -i test/cases -o test/fuzz-output -- ./zig-out/bin/arofuzz
 ```
+---
+## Running fuzz tests via Zig build system
+```sh-session
+zig build test-unit --fuzz
+```