test: env: Enable env unit tests by default

If CONFIG_UNIT_TEST is enabled we should enable the individual tests by
default to ensure good test coverage.
Reviewed-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

test/env/Kconfig

 config UT_ENV
 	bool "Enable env unit tests"
 	depends on UNIT_TEST
+	default y
 	help
 	  This enables the 'ut env' command which runs a series of unit
 	  tests on the env code.