Tests Fail to Compile

The `Makefile` seems to attempt to compile a non-existing file.

Specifically, the `all` target includes `test_attribute`, but there is no corresponding `test_attribute.cpp` (or other source file) in the `tests/` directory:

https://github.com/HDFGroup/vol-cache/blob/86c9e71a04752bb76a4c98450f858fa2bee21172/tests/Makefile#L21C1-L22C51

As a result, running `make` in `tests/` fails with:

make: *** No rule to make target 'test_attribute.o', needed by 'test_attribute'. Stop.

I'm assuming this source file was removed but the `Makefile` was not updated. Removing `test_attribute` from the `all` target allows the remaining tests to build successfully.