refactor(logging): init Builder location_ in constructor; add location tests

Initialize location_ in the Builder constructor instead of via a default
member initializer (behavior-preserving). Add tests pinning the default
location to the constructor site and the Location() override to the caller's
file and line.

Co-authored-by: Isaac

Author: kamcheungting-db

src/iceberg/logging/logger.h

@@ -85,7 +85,8 @@ struct ICEBERG_EXPORT LogMessage {
 /// Location() (e.g. to forward a caller's std::source_location).
 class ICEBERG_EXPORT LogMessage::Builder {
  public:
-  explicit Builder(LogLevel level) : level_(level), location_(std::source_location::current()) {}
+  explicit Builder(LogLevel level)
+      : level_(level), location_(std::source_location::current()) {}
 
   /// \brief Set the already-formatted message text.
   Builder& Message(std::string message) {