TAP tests aren't using the magic words for Windows file access

Buildfarm member drongo has been failing the pg_ctl regression test
pretty often.  I happened to look closer at what's happening, and
it's this:

could not read
"C:/prog/bf/root/HEAD/pgsql.build/src/bin/pg_ctl/tmp_check/t_004_logrotate_primary_data/pgdata/current_logfiles":
Permissiondenied at C:/prog/bf/root/HEAD/pgsql.build/src/test/perl/TestLib.pm line 397. 

That is, TestLib::slurp_file is failing to read a file.  Almost
certainly, "permission denied" doesn't really mean a permissions
problem, but failure to specify the file-opening flags needed to
allow concurrent access on Windows.  We fixed this in pg_ctl
itself in commit 0ba06e0bf ... but we didn't fix the TAP
infrastructure.  Is there an easy way to get Perl on board
with that?

            regards, tom lane