Prevent 006_transfer_modes.pl from leaving files behind.

Prevent 006_transfer_modes.pl from leaving files behind.

This test was leaving files like delete_old_cluster.{sh,bat} in the
source directory for VPATH and meson builds.  To fix, change the
directory to tmp_check before running the test, as was done in
commits 15b6d21553, 8af917be6b, and c462b054ba.

Oversight in commit af0d4901c1.

Reported-by: Andrew Dunstan <[email protected]> (on Discord)
Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Andrew Dunstan <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/Z_RHkG770w3SE0yU%40nathan