Fix tablespace path TAP test of pg_verify_checksums for msys

Fix tablespace path TAP test of pg_verify_checksums for msys

TAP tests on msys need to run with the DTK perl, which understands msys
virtualized paths.  Postgres, however, does not understand such paths,
so before a path can be used safely with CREATE TABLESPACE, it needs to
be translated into a path on the underlying file system.

Per report from buildfarm member jacana.  Suggested fix is from Andrew
Dunstan.

Discussion: https://postgr.es/m/20181130053555.GF2267@paquier.xyz