Deal with paths containing \ and spaces in basebackup_to_shell tests

Deal with paths containing \ and spaces in basebackup_to_shell tests

As $gzip is embedded in postgresql.conf \ needs to be escaped, otherwise guc.c
will take it as a string escape. Similarly, if "$gzip" contains spaces, the
prior incantation will fail. Both of these are common on windows.

Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/ce1b6eb3-5736-6f38-9775-b7020128b8d8@enterprisedb.com
Backpatch: 15-, where the test was added in 027fa0fd726