pg_rewind tests

There are some small issues with the pg_rewind tests.

This technique

check: all
$(prove_check) :: local
$(prove_check) :: remote

for passing arguments to "prove" does not work with the tools included
in Perl 5.8.

While sorting out the portability issues in the TAP framework during the
9.4 release cycle, we had set 5.8 as the oldest Perl version that is
supported. (It's the Perl version in RHEL 5.) I suggest using
environment variables instead, unless we want to change that.

Moreover,

if ($test_mode == "local")
...
elsif ($test_mode == "remote")

don't work, because those are numerical comparisons, not string
comparisons. So the remote branch is never actually run.

Finally, RewindTest.pm should use

use strict;
use warnings;

and the warnings caused by that should be addressed.