tests: fdisk: add regression test for missing EBR gap between logical partitions

tests: fdisk: add regression test for missing EBR gap between logical partitions

Add a DOS/MBR test case where the first logical partition starts at 6145
(extended start 2048). In the regressed behaviour fdisk allows the next
logical partition to start at prev_end+1, which would place its EBR
(start - 2048) inside the previous logical partition.

The expected behaviour is to require the next starting sector to be at least
prev_end + 2048 + 1 (e.g. 790528 in the testcase).

Closes issue #3925.