Native support for unit testing in Leo

[howardwu]

No description provided.

[collinc97]

Proposed testing syntax:

Use the test keyword to add tests to a leo program. Tests must have 0 function inputs and 0 function returns.

function main(a: u32) -> u32 {
    return a
}

test function expect_pass() {
  let a = 1u32;

  let res = main(a);

  assert_eq!(res, 1u32);
}

test function expect_fail() {
  assert_eq!(1u8, 0u8);
}