setup spirv backend in behavior tests

test/behavior/pub_enum.zig

@@ -1,7 +1,10 @@
+const builtin = @import("builtin");
 const other = @import("pub_enum/other.zig");
 const expect = @import("std").testing.expect;
 
 test "pub enum" {
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
     try pubEnumTest(other.APubEnum.Two);
 }
 fn pubEnumTest(foo: other.APubEnum) !void {