New timing unit tests

New set of unit tests for the timing module, instead of just running
the selftest function.

The selftest function sometimes fails on a heavily loaded
machine (such as a typical continuous integration system). Because of
the all-in-one nature of the test and because the exact load pattern
can be hard to reproduce, it is difficult to diagnose failures of CI
runs with selftest. The new tests are more separated and I strove to
point out potential failure modes in comments.

* mbedtls_timing_hardclock: not tested. This function gives so few
  guarantees that there isn't much to test, and it is hard to test
  reliably because clock cycles don't easily relate to time in any
  remotely portable way. This function isn't used in the library
  anyway, it's only there for benchmark programs.
* mbedtls_timing_get_timer: tested by setting a timer and verifying
  that it reaches its target, and by verifying that a timer started
  later than another always has a smaller elapsed time.
* mbedtls_set_alarm: tested by setting an alarm, busy-waiting for it
  and measuring the elapsed time with a timer.
* mbedtls_timing_set_delay, mbedtls_timing_get_delay: tested by
  setting a delay object and watching it go through its two delay
  values, using a timer to check that the delays are passed at the
  expected time.

The tests pass under light to moderate load, but some of them can be
defeated with sufficiently heavy load. This is unavoidable since the
test process to be effectively suspended for any length of time,
making us think that a timer has gone on for too long.

ChangeLog

@@ -6,6 +6,7 @@ Features
    * Allow comments in test data files.
    * The selftest program can execute a subset of the tests based on command
      line arguments.
+   * New unit tests for timing.
 
 Bugfix
    * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.