tests: utils.h: forbid compilation with NDEBUG

This allows to rely on the assert() macro being functional on
the test suite.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>
15 jobs from master in 50 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Stage1 Testing
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FIPS140-2.Fedora.x86_64

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MinGW32

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abi-coverage.Fedora

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asan.Fedora.x86_64

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doc-dist.Fedora

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minimal.Fedora.x86_64

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no-SSL-3.0.Fedora.x86_64

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static-analyzers.Fedora.x86_64

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ubsan-Werror.Fedora.x86_64

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valgrind.Fedora.x86_64

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