Remove oneline= parameter from utils.ReadFile

This partially reverts commit b774bb10.
Unittests unrelated to that particular functionality but introduced in
that commit are left untouched. Since the temporary directory is now
only used to check for a non-existing file, it is removed as well, and
a convenient very-very-unlikely-to-exist file is used instead.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>