databases/galera: Split compilation from tests run

Previously tests were ran intermixed with compilation commands in
parallel (scons -jN). Running tests in parallel produces some sporadic
failures which would make the compilation fail.

With this change, when compiling ("make" in the ports directory)
"scons -j8 ... tests=0" is used to request compilation only.

To run the tests do "make test" in the ports directory. It will run
"scons ... tests=1" (without -jN) and will only run the tests.