Make DBD::Firebird work out of the box with OpenSUSE on perl-5.18.2-ld

• ibase.h is installed in /usr/include/firebird from the firebird-devel package, but that is not found, as /usr/include is a valid folder, so the alternatives are not checked
• It is legal to have an empty password. The empty sting however evaluated to false in your tests
• Allow FIREBIRD_DATABASE to set the database to test on, as File::Spec->tmpdir is not always allowed in firebird configuration
• Test with predicatable float/double values, as those tests will otherwise fail on perl built with longdouble support
• Test all attributes (I simplified the FETCH function a little bit, and I note that your XS code might not be thread-safe. I miss dTHX and aTHX in there). The test case now uses is_deeply and tests all supported attributes (I think)