bitfield: add tests

Add tests for the bitfield helpers. The constant ones will all
be folded to nothing by the compiler (if everything is correct
in the header file), and the variable ones do some tests against
open-coding the necessary shifts.

A few test cases that should fail/warn compilation are provided
under ifdef.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 8838d11..d3d82ec 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1802,6 +1802,13 @@
 
 	  If unsure, say N.
 
+config TEST_BITFIELD
+	tristate "Test bitfield functions at runtime"
+	help
+	  Enable this option to test the bitfield functions at boot.
+
+	  If unsure, say N.
+
 config TEST_UUID
 	tristate "Test functions located in the uuid module at runtime"