Support absolute bindir/libdir in regression tests with meson

Support absolute bindir/libdir in regression tests with meson

Passing an absolute bindir/libdir will install the binaries and
libraries to <build>/tmp_install/<bindir> and
<build>/tmp_install/<libdir> respectively.

This path is correctly passed to the regression test suite via
configure/make, but not via meson, yet. This is because the "/"
operator in the following expression throws away the whole left side
when the right side is an absolute path:

    test_install_location / get_option('libdir')

This was already correctly handled for dir_prefix, which is likely
absolute as well. This patch handles both bindir and libdir in the
same way - prefixing absolute paths with the tmp_install path
correctly.

Author: Wolfgang Walther
Reviewed-by: Nazir Bilal Yavuz, Alvaro Herrera, Peter Eisentraut
Reviewed-by: Tristan Partin
Discussion: https://www.postgresql.org/message-id/ca8f37e1-a2c3-40e2-91f6-59c3d3652ad4@technowledgy.de
Backpatch: 16-, where meson support was added