commit: Improve style and add tests for 'built-in-symbol-file'.

[Stephen J. Turnbull]

Stephen Turnbull writes:

 > changeset:   4381:3906442b491b42211630c8b8cd8a8cc05060ce4a
 > tag:         tip
 > user:        Stephen J. Turnbull <stephen at xemacs.org>
 > date:        Thu Jan 03 01:33:59 2008 -0800
 > files:       src/ChangeLog src/doc.c tests/ChangeLog tests/automated/symbol-tests.el
 > description:
 > Improve style and add tests for 'built-in-symbol-file'.

It turns out that although Aidan's test !NILP does not have a multiple
evaluation problem, at least in some configurations SUBRP does.  So
it's a bad idea to have something like (fun = Fcdr_safe (fun)) as a
macro argument, and it would be especially bad if somebody decided to
make the "obvious" optimization of moving the assignment into the
SUBRP invocation and eliminate the !NILP check altogether in that way.

I probably would have left it alone, but I was curious so wrote some
tests and decided to add them to the test suite.