[IIO] Added data validity testcases

[IIO] Added data validity testcases

1. Added test case to check data validity for the element for various data bytes sizes
2. Added a property of timeout while reading file in non-blocking mode.
This allows looping while reading the file along with EAGAIN error.
3. Corrected mask generation
4. IIO dev file is replaced with fifo. Using threads to make these fifo as
opening it blocks unless both read and write mode are opened together.
5. Added test case to validate data with and without trigger.

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>