tests: look for gpio-tools in more places

tests: look for gpio-tools in more places

Currently we only support running the test cases for gpio-tools from
the top-level source directory. Some users want to install the test
executable and run the tests from other locations (/bin, /usr/bin or
custom path).

This patch makes the test suite look in the source tree path first,
then check the directory in which the program resides and last iterate
over all directories in $PATH.

We only do that once at the beginning and then reuse the path later.

Cc: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Anders Roxell <anders.roxell@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>