Agreement test for strings:

    check_both("str_len(\"hello\")", 5);
    check_both("str_len(\"\")", 0);
    check_both("store8(100, 65); load8(100)", 65);

And the big one -- streq_mem running through both engines:

    check_both(
        \\fn streq_mem(a: i64, b: i64) i64 {
        \\    var a_addr: i64 = a / 65536;
        \\    var a_len: i64 = a - a_addr * 65536;
        \\    var b_addr: i64 = b / 65536;
        \\    var b_len: i64 = b - b_addr * 65536;
        \\    if (a_len != b_len) { return 0; }
        \\    var i: i64 = 0;
        \\    while (i < a_len) {
        \\        if (load8(a_addr + i) != load8(b_addr + i)) { return 0; }
        \\        i = i + 1;
        \\    }
        \\    return 1;
        \\}
        \\streq_mem("hello", "hello")
    , 1);

If this passes -- a string comparison function, written in our language, compiled to WAT, executed by the VM, producing the same result as the interpreter -- then the language is powerful enough to do real string processing. It can compare keywords. It can check operators. It can parse.