[MCX] Fix compilation of Talpa on MCX.

MCX's old compiler doesn't like using #ifdef in the middle of a likely() macro.
Skip running NULL device mount test if the kernel itself doesn't support that, since otherwise syscall hook is left with a bad use count.
Move NULL device mount test earlier to get failures earlier.
Make runTest work without a source control checkout.

[+RS]

[git-p4: depot-paths = "//dev/talpa/main/": change = 2407488]
Fix NULL mnt unit test to not use /mnt

[+RS]

[git-p4: depot-paths = "//dev/talpa/main/": change = 2408186]