contracts: Remove wat support from test fixtures (#3588)

In order to prepare for PolkaVM support I removed the wat support from
our test fixture crate.

- Removed redundant tests (invalid module checks are already inside the
prepare module where they belong
- Converted the gas_sync tests to Rust
- Moved the start function test to the `wasm` module

substrate/frame/contracts/src/wasm/mod.rs

@@ -3442,4 +3442,22 @@ mod tests {
 			runtime.read_sandbox_memory_as(&memory, 0u32).unwrap();
 		assert_eq!(decoded.into_inner(), data);
 	}
+
+	#[test]
+	fn run_out_of_gas_in_start_fn() {
+		const CODE: &str = r#"
+(module
+	(import "env" "memory" (memory 1 1))
+	(start $start)
+	(func $start
+		(loop $inf (br $inf)) ;; just run out of gas
+		(unreachable)
+	)
+	(func (export "call"))
+	(func (export "deploy"))
+)
+"#;
+		let mut mock_ext = MockExt::default();
+		assert_err!(execute(&CODE, vec![], &mut mock_ext), <Error<Test>>::OutOfGas);
+	}
 }