test: disable 'switch on large types' behavior tests for wasm backend

The self-hosted wasm backend doesn't properly support very large integers yet.

test/behavior/switch.zig

@@ -1459,6 +1459,8 @@ test "switch on nested packed containers" {
 }
 
 test "switch on large types" {
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest;
+
     const S = struct {
         fn doTheTest(a: u128, b: i500) !void {
             switch (a) {

test/behavior/switch_loop.zig

@@ -566,6 +566,8 @@ test "switch loop with packed unions with OPV" {
 }
 
 test "switch loop on large types" {
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest;
+
     const S = struct {
         fn doTheTest(a: u128, b: i500) !void {
             label: switch (a) {