Cover additional errors and corner conditions in repack.c

Cover additional errors and corner conditions in repack.c

The coverage report shows that some error cases were not being tested;
add test cases for them.

While at it, move some recently added ones to the test_decoding suite:
the preventative check added in 43649b6a53e9 now causes servers with
wal_level=minimal to error out earlier than before.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Baji Shaik <baji.pgdev@gmail.com>
Discussion: https://postgr.es/m/ahiwD29RNfVT4tjQ@alvherre.pgsql