Remove fl_julia_current_{file, line} (#53797)
These flisp accessor functions make use of the `jl_filename` and
`jl_lineno` globals. I was looking at removing these globals for
unrelated reasons, when I saw that one of the primary uses was in these
flisp accessor, which appear entirely unnecessary. They are only used to
provide a default for an error message, but the place that puts the
error message into a list to return to julia does actually already have
access to this information, so there's no need for these to look at the
globals.
While we're at it, also add a test for this code path, which was
otherwise unexercised in our test suite.
