Tidy tests a bit. (#6122)

substrate/frame/contracts/fixtures/check_default_rent_allowance.wat

@@ -0,0 +1,47 @@
+(module
+	(import "env" "ext_rent_allowance" (func $ext_rent_allowance))
+	(import "env" "ext_scratch_size" (func $ext_scratch_size (result i32)))
+	(import "env" "ext_scratch_read" (func $ext_scratch_read (param i32 i32 i32)))
+	(import "env" "memory" (memory 1 1))
+
+	(func $assert (param i32)
+		(block $ok
+			(br_if $ok
+				(get_local 0)
+			)
+			(unreachable)
+		)
+	)
+
+	(func (export "call"))
+
+	(func (export "deploy")
+		;; fill the scratch buffer with the rent allowance.
+		(call $ext_rent_allowance)
+
+		;; assert $ext_scratch_size == 8
+		(call $assert
+			(i32.eq
+				(call $ext_scratch_size)
+				(i32.const 8)
+			)
+		)
+
+		;; copy contents of the scratch buffer into the contract's memory.
+		(call $ext_scratch_read
+			(i32.const 8)		;; Pointer in memory to the place where to copy.
+			(i32.const 0)		;; Offset from the start of the scratch buffer.
+			(i32.const 8)		;; Count of bytes to copy.
+		)
+
+		;; assert that contents of the buffer is equal to <BalanceOf<T>>::max_value().
+		(call $assert
+			(i64.eq
+				(i64.load
+					(i32.const 8)
+				)
+				(i64.const 0xFFFFFFFFFFFFFFFF)
+			)
+		)
+	)
+)