Incorrect parsing of RHS of binary expression when it looks like a flag

[lincheney]

The following
[ a = -d ]
is incorrectly parsed as

program [0, 0] - [1, 0]
  test_command [0, 0] - [0, 10]
    binary_expression [0, 2] - [0, 10]
      left: word [0, 2] - [0, 3]
      right: unary_expression [0, 6] - [0, 10]
        test_operator [0, 6] - [0, 8]
        word [0, 9] - [0, 10]
    MISSING ] [0, 10] - [0, 10]

Notably the -d in this context is a string literal, but is being treated as the the test_operator in a unary_expression (which also swallows the ] as the unary operand, resulting in the incorrect MISSING ] at the end)