Don't clean initdb files on template creation failure

Don't clean initdb files on template creation failure

Commit 252dcb32397f6 introduced initdb template caching to speed up
tests by re-using initdb output.  The initdb command didn't however
use the --no-clean option to preserve generated data in case initdb
crashes unlike pg_regress which does do this.  This adds the option
to initdb to aid debugging.

While changing the commandline, switch to using long options for
initdb to make the code more self-documenting.

Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/CAEze2WhSTjfK_M+Ea4GSQp8odrEOaQS8HyORd1TJUEiyXaB+rw@mail.gmail.com