[tests] use constexpr for constants in unit tests (#13145)

This commit refactors various unit tests to use `constexpr` for
defining constants instead of anonymous `enum` types.

Using `constexpr` is the modern and preferred approach in C++, as it
provides explicit types for constants and improves code clarity and
type safety.

tests/unit/test_spinel_encoder.cpp

@@ -35,10 +35,7 @@
 namespace ot {
 namespace Spinel {
 
-enum
-{
-    kTestBufferSize = 800,
-};
+constexpr uint16_t kTestBufferSize = 800;
 
 otError ReadFrame(Spinel::Buffer &aNcpBuffer, uint8_t *aFrame, uint16_t &aFrameLen)
 {