I chanced to notice while thumbing through lcov reports that we had
exactly no coverage of BETWEEN SYMMETRIC, nor of current_time(N) and
localtime(N).  Improve that.

parse_expr.c still has a pretty awful coverage number, but a large part
of that is due to lack of coverage of the operator_precedence_warning
logic.  I have zero desire to write tests for that; I think ripping it
out would be more sensible at this point.