test: Add E2E test coverage for shader IO

This provides much more complete coverage than what we have in the
unit tests. We now test:
- All builtins, for all stages, both struct and non-struct
- Multiple location attributes for vertex and fragment stages, both
  struct and non-struct
- Mixing builtins and location attributes, whilst mixing struct and
  non-struct
- A few "interesting" cases of IO structs being shared between
  different functions, stages, and with an SSBO variable

There are 7 skipped tests for MSL due to two different MSL bugs which
will be fixed in upcoming patches.

Change-Id: I8b802591762c8ff018e01bf37838551e353162b1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53120
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>

test/shader_io/shared_struct_helper_function.wgsl

@@ -0,0 +1,16 @@
+struct VertexOutput {
+  [[builtin(position)]] pos : vec4<f32>;
+  [[location(0)]] loc0 : i32;
+};
+
+fn foo(x : f32) -> VertexOutput {
+  return VertexOutput(vec4<f32>(x, x, x, 1.0), 42);
+}
+
+fn vert_main1() -> VertexOutput {
+  return foo(0.5);
+}
+
+fn vert_main2() -> VertexOutput {
+  return foo(0.25);
+}