build: include tests/Makefile if available

Testing OpenWrt is important, and there is a test suite in the making.
For maximum convenience and minimal CI over-usage, make it simple to run
tests locally. The main Makefile now attempts to include
`tests/Makefile` and silently fails if it doesn't.

While the test suite[1] is still young, it provides good examples of how
to test things around OpenWrt: starting with shell scripts using
`bats`[2], followed by QEMU tests, and finally real device tests using
LabGrid[3]. This could lead to the creation of the best OpenWrt version
yet.

Please consult the `openwrt-tests.git` README.md for details on the
setup. Once installed you may run commands like the following:

* make tests/shell  # run shell tests
* make tests/x86-64 # run  and test x86/64 in QEMU

[1]: http://github.com/aparcar/openwrt-tests/
[2]: https://bats-core.readthedocs.io
[3]: https://labgrid.readthedocs.io



Signed-off-by: Paul Spooren <mail@aparcar.org>
Link: https://github.com/openwrt/openwrt/pull/15647


Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit a8ff0c1b7e5aefb8bcd31e0eeb82d2c11399223f)