tests: elfcopy --reverse-offs should only swap sections next to each other.

tests: elfcopy --reverse-offs should only swap sections next to each other.

The run-reverse-sections-self.sh (which really should have been called
"swap-sections") could fail if the ELF file had sections that were not
directly next to each other swapped. Add a check to make sure the swapped
sections are actually directly next to each other.

Signed-off-by: Mark Wielaard <mark@klomp.org>