build: enable compiling tests not in debug mode (and without code coverage)

Below is the type of compilation error I was getting:

$ cmake -G Ninja ../.. -DPython_EXECUTABLE=python3.7 -DBUILD_TESTS=ON -DENABLE_COVERAGE=ON -DBUILD_CPP_DOCS=ON -DBUILD_PYTHON_DOCS=ON
...
$ ninja
[54/174] Building CXX object producer/api/cpp/CMakeFiles/test-asapo-producer.dir/unittests/test_producer_impl.cpp.o
FAILED: producer/api/cpp/CMakeFiles/test-asapo-producer.dir/unittests/test_producer_impl.cpp.o
ccache /usr/bin/c++  -I/home/marc-olivier/Documents/desy/dev/gitlab.desy.de/asapo/asapo/producer/api/cpp/include -I/home/marc-olivier/Documents/desy/dev/gitlab.desy.de/asapo/asapo/producer/api/cpp/../c/include -I/home/marc-olivier/Documents/desy/dev/gitlab.desy.de/asapo/asapo/common/cpp/include -g -O0 --coverage -fprofile-arcs -ftest-coverage -Wall -Wextra -pedantic -Wconversion -Wno-missing-field-initializers -fPIC -DGTEST_HAS_PTHREAD=1 -std=gnu++11 -MD -MT producer/api/cpp/CMakeFiles/test-asapo-producer.dir/unittests/test_producer_impl.cpp.o -MF producer/api/cpp/CMakeFiles/test-asapo-producer.dir/unittests/test_producer_impl.cpp.o.d -o producer/api/cpp/CMakeFiles/test-asapo-producer.dir/unittests/test_producer_impl.cpp.o -c /home/marc-olivier/Documents/desy/dev/gitlab.desy.de/asapo/asapo/producer/api/cpp/unittests/test_producer_impl.cpp
In file included from /home/marc-olivier/Documents/desy/dev/gitlab.desy.de/asapo/asapo/producer/api/cpp/unittests/test_producer_impl.cpp:15:
/home/marc-olivier/Documents/desy/dev/gitlab.desy.de/asapo/asapo/producer/api/cpp/unittests/mocking.h:20:14: error: ‘uint64_t asapo::MockDiscoveryService::UpdateFrequency()’ marked ‘override’, but does not override
   20 |     uint64_t UpdateFrequency() override {
      |              ^~~~~~~~~~~~~~~