build: fix tests build when -NDEBUG is passed

This should prevent build failures by disabling -NDEBUG for tests
that require assert by setting -UNDEBUG.

Some buildsystems like buildroot pass -NDEBUG to meson depending
on the build configuration, instead of failing for tests we should
just override that cflag for tests so that the build does not fail.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
5 jobs for fix-ndebug in 2 minutes and 52 seconds (queued for 17 seconds)
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  Container Prep
passed container_prep #10872347

00:00:24

 
  Build
passed build-docs #10872349
kvm

00:01:18

passed build-full #10872348
kvm

00:01:58

passed build-no-gl #10872350
kvm

00:01:14

 
  Pages
passed docs-and-coverage #10872351

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