bpf: Update selftests to reflect new error states

Update various selftest error messages:

 * The 'Rx tried to sub from different maps, paths, or prohibited types'
   is reworked into more specific/differentiated error messages for better
   guidance.

 * The change into 'value -4294967168 makes map_value pointer be out of
   bounds' is due to moving the mixed bounds check into the speculation
   handling and thus occuring slightly later than above mentioned sanity
   check.

 * The change into 'math between map_value pointer and register with
   unbounded min value' is similarly due to register sanity check coming
   before the mixed bounds check.

 * The case of 'map access: known scalar += value_ptr from different maps'
   now loads fine given masks are the same from the different paths (despite
   max map value size being different).

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>