Fix pg_waldump/t/001_basic.pl with BSD tar on ZFS.

Fix pg_waldump/t/001_basic.pl with BSD tar on ZFS.

The new test fails with an error about a missing WAL file on that
stack, because it is archived in GNU tar's --sparse --format=posix
format.  BSD tar uses that format by default, unlike GNU tar itself, and
ZFS triggers it by implicitly creating sparse files when it sees a lot
of zeroes.

The problem will surely also affect real users of the new tar support in
pg_waldump (commit b15c1513) and pg_verifybackup (commit b3cf461b3) on
such systems.  Ideas under discussion, but for now the test is made to
pass by disabling sparse file detection in BSD tar.

Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/1624716.1774736283%40sss.pgh.pa.us