Fixed an issue with assignment operators and expressions

- Assignment operators would not work correctly, or at all, when
  followed by an expression (as opposed as a value).
- Fixes bugreport:7864, thanks to Lelouch, Ind.
- Added associativity tests for those operators to the self-test script.
- Added total error count to the self-test script (thanks to Lighta.)

Signed-off-by: Haru <haru@dotalux.com>