parser: port test "sentinel-terminated slice type"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

parser_test.zig

@@ -1817,6 +1817,15 @@ test "zig fmt: c pointer type" {
     );
 }
 
+test "zig fmt: sentinel-terminated slice type" {
+    try testCanonical(
+        \\pub fn toSlice(self: Buffer) [:0]u8 {
+        \\    return self.list.toSlice()[0..self.len()];
+        \\}
+        \\
+    );
+}
+
 test "zig fmt: top-level tuple function call type" {
     try testCanonical(
         \\foo()