[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_util.cpp

@@ -32,10 +32,7 @@
 
 void DumpBuffer(const char *aTextMessage, const uint8_t *aBuffer, uint16_t aBufferLength)
 {
-    enum
-    {
-        kBytesPerLine = 16, // Number of bytes per line.
-    };
+    static constexpr uint16_t kBytesPerLine = 16; // Number of bytes per line.
 
     char     charBuff[kBytesPerLine + 1];
     uint16_t counter;