I agree that it would be nice to have a feature like this.  However, since multiple functions may be defined in a .oct file or a .cc file that is built in to Octave, it seems like it would be likely to mislead users into thinking that there are many more tests for built-in functions than are actually present.  I'd rather not install this change unless we have a way to tag a block of tests in a source file as belonging to a particular function and can also make the test function run only those tests for the given function, not all tests from the associated source file.

It should be relatively straightforward to tag the comment blocks with something like

%!tests-for: fcn-name
%! [...]
%!end tests-for

I don't know exactly what the syntax should be, but the point is to be able to mark a group of functions as belonging to a particular function.  Then instead of looking up the file name with "which", we could just arrange for Make to create -tst files for each function from the source files.  Then we might be able to just have the test function look for -tst files with the same name as the function.

I also don't think that we should be putting the logic of make into the test function.  Let make manage the job of rebuilding the -tst files.

The assumption that the source directory is in ".." is too specific.  The build and source trees don't have to be nested or related.

Very small note, instead of

exist (__name) == 5

which searches everything and may or may not return 5, we can limit the search to builtin functions right from the start:

exist (__name, "builtin")

which is also more readable.

Trying to test an individual built in function gives something like

>> test min
????? min is a built-in function

The attached patch allows this to work when run from the build directory (either through ./run-octave or, I presume, after installing).  It simply checks if the .cc-tst file corresponding to  which min  exists, and if so, tests it.