tests: Add option to time the S2K function.
* tests/t-kdf.c: Include stopwatch.h. (dummy_consumer): new. (bench_s2k): New. (main): Add option parser and option --s2k.
For example:
$ ./t-kdf --s2k 1765990488.0ms
$ ./t-kdf --s2k 65536 0.3ms
This test is similar to the code done by gpg-agent to calibrate the
S2K count.