pg-tle-test: extend to the 0.9.6 -> current update path via DESTDIR sandboxing

[jnasbyupgrade]

pgxntool 2.3.0's installcheck-ordering fix (pgxntool#83) made installcheck
(and so make test) unconditionally depend on install, which writes a real
.control file to disk - defeating the whole point of a pg_tle-only-deployment
proof, and with no upstream fix yet (pgxntool#90). bin/test_existing's
run_suite gains a TEST_EXISTING_DEPLOY=pgtle mode that instead redirects
make test's install step through a scratch DESTDIR (harmless here since a
pg_tle-deployed database never needs those files), pre-seeding a stub
pgtap.control at the DESTDIR-prefixed path pgxntool's pgtap prerequisite
checks for - otherwise its DESTDIR-blind recipe (pgxn install pgtap --sudo)
would trigger a real, unwanted reinstall against the scratch dir.

Also fixes test/install/load.sql's existing-mode assertion, which cross-checks
the installed version against pg_available_extensions.default_version -
that view never sees pg_tle registrations (it only reads .control files off
disk), so it would return NULL under a pg_tle deployment even though CREATE
EXTENSION resolves correctly through pg_tle's own catalog. A new
count_nulls.test_existing_deploy GUC (driven by the same TEST_EXISTING_DEPLOY
var, Makefile-exported like TEST_SCHEMA/TEST_LOAD_SOURCE) selects
pgtle.available_extensions() instead when deploy=pgtle.

The pg-tle-test CI job now installs count_nulls at 0.9.6 into a second
scratch database (inheriting the job's template1 pg_tle registration),
plants + proves the dependency guard, ALTER EXTENSION UPDATEs to current,
then runs the real pgTAP suite against it via the new mode - with
bin/assert_fs_clean verify checked after every one of those steps, not just
at the end, since that's what actually proves the DESTDIR sandboxing worked.

Verified locally (no real pg_tle in this container): confirmed via
mtime comparison that the real count_nulls.control and pgtap.control are
untouched by a full prepare-old/update/run-suite(pgtle) cycle, that the
scratch DESTDIR is cleaned up on both success and a forced regression
failure (EXIT trap surviving set -e), and exercised the
count_nulls.test_existing_deploy branch in load.sql against a stubbed
pgtle.available_extensions() function. The genuine pg_tle-registered
version needs a real CI run to confirm end to end.

🤖 Generated with Claude Code

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