Enable build-time tests

[Andreas Hasenack]

The build-time tests are disabled by default, and have to be enabled explicitly.

From the README file:
"""
## Tests

libyang includes several tests built with [cmocka](https://cmocka.org/). The tests
can be found in `tests` subdirectory and they are designed for checking library
functionality after code changes. Additional regression tests done with
a corpus of fuzzing inputs that previously caused crashes are done.
Those are available in `tests/fuzz` and are built automatically with the
cmocka unit tests.

The tests are by default built in the `Debug` build mode by running
```
$ make
```

In case of the `Release` mode, the tests are not built by default (it requires
additional dependency), but they can be enabled via cmake option:
```
$ cmake -DENABLE_TESTS=ON ..
```
"""

I suggest the last approach, to keep the build type "Release".

I actually tried that, and hit a bug[1], for which upstream committed a fix[2] to the devel branch.

1. https://github.com/CESNET/libyang/issues/1773
2. https://github.com/CESNET/libyang/commit/de2e8b272b8238258f36c42386f3f2c9806026fd

[Launchpad Janitor (janitor)]

This bug was fixed in the package libyang2 - 2.0.112-6ubuntu1

---------------
libyang2 (2.0.112-6ubuntu1) jammy; urgency=medium

  * Enable build time tests (LP: #1958385):
    - d/rules: set -DENABLE_TESTS=ON
    - d/p/fix-test-suite-wrt-FILE.patch: fix test suite failure due
      to __FILE__ being a relative path

 -- Andreas Hasenack <email address hidden> Thu, 20 Jan 2022 21:03:40 +0000