Set custom env variables for unit tests

This patch adds a way to export custom environment variables to unit
tests from qmake project file.

For instance, unit tests may use files located under source or build directory.
These paths are exported to the test environment by adding the following code
to qmake file.

TEST_CUSTOM_ENV += \\\"top_srcdir="$$top_srcdir"\\\"
TEST_CUSTOM_ENV += \\\"top_builddir="$$top_builddir"\\\"

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Update documentation for custom env

Provide examples on how to use custom env.

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Merge pull request #1 from giraldeau/custom-env

Custom environment variables