pgsql: pg_validatebackup: Adjust TAP tests to placate perlcritic.

pg_validatebackup: Adjust TAP tests to placate perlcritic.

It seems that we have a policy that every Perl subroutine should
end with an explicit "return", so add explicit "return"
statements to all the new subroutines added by my prior
commit 0d8c9c1210c44b36ec2efcb223a1dfbe897a3661.

Per buildfarm.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/87e300434058a157bbc4ef8d039937abdefa7610

Modified Files
--------------
src/bin/pg_validatebackup/t/003_corruption.pl   | 13 +++++++++++++
src/bin/pg_validatebackup/t/005_bad_manifest.pl |  3 +++
2 files changed, 16 insertions(+)