runtests: Make path of time binary configurable

I work on a platform that does not come with a time program. This patch
makes it possible to specify the path of the time binary or not use it
if none is available.

If the URCU_TEST_TIME_BIN environment variable exists and is executable,
it is used. Otherwise it tries with /usr/bin/time, the most common
location. If it is not there, the tests are ran without timing info.

[ Edit by Mathieu Desnoyers: use "." instead of "source" (no bash-ism),
  edit commit about check for emptiness vs definition to match the code. ]

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>