test: [naga wgsl-in] add tests for @must_use

naga/tests/in/5186-must-use.wgsl

@@ -0,0 +1,53 @@
+fn main() { }
+
+@must_use
+fn a() -> i32 { return 10; }
+
+@must_use
+fn b() -> bool { return false; }
+
+// use as return
+fn z() -> i32 {
+    return a();
+}
+
+// use in expression
+fn y() -> bool {
+    return a() == 10;
+}
+
+// use in short circuited expression
+fn x() -> bool {
+    return false && a() == 10;
+}
+
+// assignment and use as return
+fn w() -> i32 {
+    let called_a = a();
+    return called_a;
+}
+
+// assignment and use before return
+fn v() -> i32 {
+    let called_a = a();
+    let used_a = 1 + called_a;
+    return used_a;
+}
+
+// use in condition
+fn u() -> i32 {
+    if(b()) {
+        return 1;
+    } else {
+        return 2;
+    }
+}
+
+// use in skipped branch
+fn t() -> i32 {
+    if(true) {
+        return 1;
+    } else {
+        return a();
+    }
+}