Spotted by Coverity: if you _just_ gave a filename to bidi_test,
without any previous argument that set testfn to something other than
NULL, the program would crash rather than giving an error message.
(It's only a test program, but test programs you only run once in a
blue moon are the ones that _most_ need to explain their command-line
syntax to you carefully, because you've forgotten it since last time
you used them!)
Also, conditionalised a memcpy on the size not being 0, because it's
illegal to pass a null pointer to memcpy _even_ if size==0. (That
would only happen with a test case containing a zero-length string,
but whatever.)
This standalone CLI program runs the UCD bidi tests in the form
provided in Unicode 14.0.0. You can run it by just saying
bidi_test --class BidiTest.txt --char BidiCharacterTest.txt
assuming those two UCD files are in the current directory.
