expr_test_utils.hh was a header file with helper methods for
expression tests. All functions were inline, because I didn't
know how to create and link a .cc file in test/boost.
Now the header is split into expr_test_utils.hh and expr_test_utils.cc
and moved to test/lib, which is designed to keep this kind of files.
Signed-off-by: Jan Ciolek <jan.ciolek@scylladb.com>
