class tests failing in VPATH builds

[Rik]

'make check' is failing for several tests in test_classes.m, but only
for VPATH builds where srcdir != builddir.  I read the comments at the
top of the file and the class test directories such as @Spork are
supposed to be in the current directory.  This is obviously not the case
for the VPATH build.  I am not very conversant with the OOP interface in
Octave, so I can suggest a few fixes, but somebody with more knowledge
should apply one.

1) Add $(srcdir) Octave's path via the --path directive or OCTAVE_PATH
variable when running the tests
2) copy over the test directories from $(srcdir) to $(builddir) using
'cp --update' so that a copy will only occur if the two directories are
different
3) run fntests.m from $(srcdir) and explicitly open any output, such as
fntests.log, in $(builddir).

--Rik