[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_string.cpp

@@ -36,10 +36,7 @@
 
 namespace ot {
 
-enum
-{
-    kStringSize = 10,
-};
+constexpr uint16_t kStringSize = 10;
 
 template <uint16_t kSize> void PrintString(const char *aName, const String<kSize> aString)
 {