Discard shader execution tests (#3467)

* Tests:
  * discard all
  * discard 3 quadrants
  * discard in function call
  * discard in various loops
  * derivatives and discard

src/webgpu/listing_meta.json

@@ -1762,6 +1762,12 @@
   "webgpu:shader,execution,stage:basic_compute:*": { "subcaseMS": 1.000 },
   "webgpu:shader,execution,stage:basic_render:*": { "subcaseMS": 1.000 },
   "webgpu:shader,execution,statement,compound:decl:*": { "subcaseMS": 29.767 },
+  "webgpu:shader,execution,statement,discard:all:*": { "subcaseMS": 36.094 },
+  "webgpu:shader,execution,statement,discard:derivatives:*": { "subcaseMS": 15.287 },
+  "webgpu:shader,execution,statement,discard:function_call:*": { "subcaseMS": 11.744 },
+  "webgpu:shader,execution,statement,discard:loop:*": { "subcaseMS": 11.821 },
+  "webgpu:shader,execution,statement,discard:three_quarters:*": { "subcaseMS": 34.735 },
+  "webgpu:shader,execution,statement,discard:uniform_read_loop:*": { "subcaseMS": 13.095 },
   "webgpu:shader,execution,statement,increment_decrement:frexp_exp_increment:*": { "subcaseMS": 4.700 },
   "webgpu:shader,execution,statement,increment_decrement:scalar_i32_decrement:*": { "subcaseMS": 20.301 },
   "webgpu:shader,execution,statement,increment_decrement:scalar_i32_decrement_underflow:*": { "subcaseMS": 4.900 },