builtins: Use commas for struct members in tests

The test files were already updated manually in a previous change, so
we just need to update the template file to match.

Bug: tint:1475
Change-Id: I6f1b3d3a7a3df014b58e67ec2645b1dfcc275de1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/85260
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/test/tint/builtins/builtins.wgsl.tmpl b/test/tint/builtins/builtins.wgsl.tmpl
index c7e2265..a5ab9ac 100644
--- a/test/tint/builtins/builtins.wgsl.tmpl
+++ b/test/tint/builtins/builtins.wgsl.tmpl
@@ -176,10 +176,10 @@
 {{-     $access  := Eval "Access"       $p.Type -}}
 {{-     if and (eq $filter_storage $storage) (eq $filter_access $access) }}
 {{-       if eq "ptr" $p.Type.Target.Name  }}
-  arg_{{$i}}: {{template "Type" (index $p.Type.TemplateArguments 1)}};
+  arg_{{$i}}: {{template "Type" (index $p.Type.TemplateArguments 1)}},
 {{          $args.Put $i (printf "&%v.arg_%v" $var_name $i) -}}
 {{-       else  }}
-  arg_{{$i}}: {{template "Type" $p.Type}};
+  arg_{{$i}}: {{template "Type" $p.Type}},
 {{          $args.Put $i (printf "%v.arg_%v" $var_name $i) -}}
 {{-       end -}}
 {{-     end -}}