Add more "weird" test cases

[Rangi42]

Things like https://github.com/rust-lang/rust/blob/master/tests/ui/weird-exprs.rs. Increase code coverage in general, which may also catch more bugs with edge cases.

[Rangi42]

The main weird test case I wanted to add was already done in 91d7ce5. RGBASM isn't a full programming language with arbitrary nesting of expressions, so fewer opportunities for weird test cases.

[Rangi42]

This works (as expected given how FOR loops are implemented), so a test case should verify that:

; prints $0 $2 $4 $3
def x = 3
for x, x
  def x *= 2
  println x
endr
println x

[Rangi42]

Added in 762e231