ADBDEV-6776 Restore PXF unit tests for external table

[dkovalev1]

PXF external table component contains a directory with unit tests. These tests
have not been updated for a while and seem to be broken now. However, ongoing
development can benefit from having unit tests ready to use.

The problem with these tests is that PXF heavily uses database internals, such
as strings, memory management, transaction states, etc. While PXF was part of
the core distribution, this was not an issue since the tests were integrated
into the GP unit test system. However, when PXF became an independent component
without references to the database source code, the problem arose. This was
perhaps the main reason the tests were abandoned.

This commit enables the tests to be built and run. However, many of the tests
remain disabled as the environment has changed significantly, and a considerable
amount of effort is still required to resurrect them. These tests have been
commented out, and compiler warnings will serve as a reminder that this work is
not yet finished.