testing/cppzmq: fix FTBFS

-- Found Threads: TRUE  
CMake Error at tests/CMakeLists.txt:51 (catch_discover_tests):
  Unknown CMake command "catch_discover_tests".


-- Configuring incomplete, errors occurred!
See also "/home/buildozer/aports/testing/cppzmq/src/cppzmq-4.7.1/build/CMakeFiles/CMakeOutput.log".
7 jobs for !16776 with testing/cppzmq in 1 minute and 40 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Lint
passed #292314
docker-alpine x86_64
lint

00:00:18

 
  Build
passed #292319
docker-alpine ci-build aarch64
build-aarch64

00:00:59

passed #292320
docker-alpine ci-build armv7
build-armv7

00:01:05

passed #292318
docker-alpine ci-build ppc64le
build-ppc64le

00:01:20

passed #292317
docker-alpine ci-build s390x
build-s390x

00:00:41

passed #292316
docker-alpine ci-build x86
build-x86

00:00:44

passed #292315
docker-alpine ci-build x86_64
build-x86_64

00:00:40