spirv: disable new behavior tests that do not pass

Some new behavior tests have recently been added, and not all of these
pass with the SPIR-V backend.

test/behavior/enum.zig

@@ -1199,6 +1199,8 @@ test "enum tag from a local variable" {
 }
 
 test "auto-numbered enum with signed tag type" {
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
     const E = enum(i32) { a, b };
 
     try std.testing.expectEqual(@as(i32, 0), @intFromEnum(E.a));