ZMMFS: Make the RAM test resiliant to stray capacitance

[hoglet67]

;; Step 3: Check if slot is RAM
.testram
        lda     &8006
        eor     #&FF
        sta     &8006
        cmp     &8006

We really need to write two locations with two different values.

See: https://stardot.org.uk/forums/viewtopic.php?p=249327#p249327

[hoglet67]

Or add a additional CMP which reads a different value from the same ROM, and perturbs the data bus.