tests: add get_double in config-parser tests

weston_config_section_get_double() was not covered with tests before.

This patch follows the testing style already present in the file.

Cannot use ZUC_ASSERT_EQ() here, because that would convert the values
to integers before comparison. Luckily, simple strict equality
comparison works here, because we are testing conversion to float, not
the results of lossy calculations.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>