Use `inline` on all namespace-scope constants defined in headers

Change-Id: Ia6e68d3fe572f6c15978c77a646ac599fa47e575
diff --git a/tests/unit/security/validator-fixture.hpp b/tests/unit/security/validator-fixture.hpp
index 7e1e0b5..39e4ed3 100644
--- a/tests/unit/security/validator-fixture.hpp
+++ b/tests/unit/security/validator-fixture.hpp
@@ -69,8 +69,8 @@
   ValidationError lastError{ValidationError::NO_ERROR};
 
 private:
-  static const time::milliseconds s_mockPeriod;
-  static const int s_mockTimes;
+  static constexpr time::milliseconds s_mockPeriod{250};
+  static constexpr int s_mockTimes{200};
 };
 
 template<class ValidationPolicyT, class CertificateFetcherT = CertificateFetcherFromNetwork>