crypto-test: Introduce first version of new .c file.

crypto-test: Introduce first version of new .c file.

This is not complete and does not compile yet.  It is not wired into
the build system.

That will come right at the end, because I have chosen not to generate
a fake history for the build machinery, which in the original
work-in-progress series underwent some churn and was then refactored
at the end.

(This commit was originally an `@@@' commit from Mark Wooding,
"@@@ crypto-test".  I have added this commit message and folded
in a couple of subsequent commits which I felt should be included.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>