Rewriting the test of pg_upgrade as a TAP test - take three - remastered set

Hi,
(Adding Andrew in CC for the buildfarm and PostgresNode parts.)

$subject has been around for a couple of years now, with the following
threads:
https://www.postgresql.org/message-id/20180126080026.GI17847@paquier.xyz
https://www.postgresql.org/message-id/CAB7nPqRdaN1A1YNjxNL9T1jUEWct8ttqq29dNv8W_o37+e8wfA@mail.gmail.com

An advantage of moving to TAP is that we can then remove the support
for upgrades within the MSVC scripts, and also remove pg_upgrade's
test.sh that has accumulated tweaks that are solved by the TAP tests,
resulting in cleanup:
8 files changed, 230 insertions(+), 403 deletions(-)

Based on the past discussions, there were two obstacles preventing to
do this switch:
- Support for tests with older versions, something where the gap as
been closed thanks to Andrew's work in 4c4eaf3d.
- Buildfarm support, and I am not sure how things need to be extended
there.

Another thing to note is that HEAD uses oldbindir, bindir and libdir
to track the location of the old and new libraries and binaries. With
the infrastructure in place, once can define only an install path for
a PostgresNode, so this version uses only two variables:
- oldinstall, for the installation path of the version to upgrade
from.
- oldsrc, to point to the source of the old version.

It is not difficult to switch to one approach or the other, but
reducing the logic to a minimum number of variables is a good deal to
take IMO.

I have been testing this patch a bit with older versions, down to 12,
and that was logically working, and PostgresNode may need more to be
able to work with ~11, as documented in get_new_node(). And I have
not tested that with MSVC yet. Anyway, attached is a new patch to
make the discussion move on. Even if there is still work to be done
here, would people here still support this switch?

Thanks,
--
Michael