tests/exynos: introduce wait_for_user_input

Currently getchar() is used to pause execution after each test.
The user isn't informed if one is supposed to do anything for
the tests to continue, so print a simple message to make this
more clear.

v3: Compactify printf calls as pointed out by
    Emil Velikov <emil.l.velikov@gmail.com>.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Joonyoung Shim <jy0922.shim@samsung.com>