Fix shader instrumentation for `wgpu_test::shader::struct_layout::uniform_input`

[ErichDonGubler]

#5046 introduced a regression to shader compilation for wgpu_test::shader::struct_layout::uniform_input that we deemed acceptable, short-term (see dded748#diff-c51c9614e4ff5936967ac762ce3e2d469375383a3c4995ad65a899ecf231b64dR232-R236).

But now, let's fix it!

[cwfitzgerald]

This is just #4371 - we aren't generating valid spirv, so shader validation is giving up before even trying.

[ErichDonGubler]

Ah, okay. I'll close in favor of that issue, then!