test: build app with meson as dpdk-test

test: build app with meson as dpdk-test

This patch enables the test/test app to be built. It also adds
the test binary to be a meson-test, which allows the meson test
infrastructure to be used to run tests.

Tests are listed using the same test binary, however each test
sets a different DPDK_TEST environment variable. The string contents
of this DPDK_TEST env var is entered in the command line interface.
As such, the familiar test names such as "ring_perf_autotest" etc
are valid tests to run using this meson test infrastructure.

Note that the tests are run serially, given that we cannot run
multiple primary processes at a time. As each test must initialize
EAL this takes some time depending on the number of hugepages.
In future, we could improve this to run multiple tests from one
EAL init, but it is out of scope for this patchset.

Finally, an option to build the tests is added to the meson build
options. When disabled, the unit test code in test/test is not
compiled. The default is set to 'true'. To disable, run:

$ meson configure -Dtests=false

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>