package/gnutls: disable tests

package/gnutls: disable tests

Tests can't build with NDEBUG is defined. Since commit 5a8c50fe05afa
("core: enable 'NDEBUG' unless BR2_ENABLE_RUNTIME_DEBUG is set") NDEBUG
is defined on !BR2_ENABLE_RUNTIME_DEBUG. Just disable tests.

Fixes:
http://autobuild.buildroot.net/results/9ec31793484a69c14a66b7338d795089c07b9937/
http://autobuild.buildroot.net/results/6ef8f1534237f2df4cff0a0bc0364fe9a46a5343/
http://autobuild.buildroot.net/results/05ff156687138627f244ca3f6cb58f1ce7d127c4/

Cc: Matt Weber <matthew.weber@collins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>