Arduino IDE - scope of the pre-processor "#define"
The scope of a #DEFINE macro is the translation unit. That is the current .c or .cpp file that is currently being compiled.
In you code PIN_NUMBER is defined in test.ino. The #include macro literaly copies the content of testLibrary.h into test.ino. So for the compiler this is working perfectly fine as PIN_NUMBER is defined.
Your testLibrary.cpp however is not happy with that. Including testLibrary.h for compilation lacks the definition of PIN_NUMBER.
The Arduino IDE is very limited and does not offer you a way to define PIN_NUMBER globally. This is usually done with the compiler argument -DPIN_NUMBER=3. All translation units will get this argument and thus the compilation would succeed. No need to define PIN_NUMBER in code.