[PATCH] Add tests for src/backend/nodes/extensible.c

Hi,

Currently extensible.c is not covered by any tests. The proposed patch
fixes this. Also it can serve as an example of using CustomScan and
ExtensibleNode.

For the reviewers
-----------------------

Here is how to check the code coverage:

```
git clean -df
rm -r build
meson setup --buildtype debug -Db_coverage=true -Dcassert=true
-Dinjection_points=true -Dtap_tests=enabled -Dldap=disabled
-Dicu=disabled -DPG_TEST_EXTRA="kerberos ldap libpq_encryption
load_balance oauth regress_dump_restore ssl wal_consistency_checking
xid_wraparound" -Dprefix=/home/eax/pginstall build
ninja -C build
meson test -C build
ninja -C build coverage-html
open build/meson-logs/coveragereport/index.html
```

You are going to need `lcov` 1.16 in your $PATH because there are
certain problems with newer versions [1].

[1]: https://postgr.es/m/CAJ7c6TN%2BMCh99EZ8YGhXZAdnqvNQYir6E34B_mmcB5KsxCB00A%40mail.gmail.com

--
Best regards,
Aleksander Alekseev