replace atan2-inputs with more meaningful inputs

replace atan2-inputs with more meaningful inputs

Commit 934d88d used inputs with exponent generated at random in the
whole binary64 exponent range, which yields essentially very large
or very small values of |y/x|. Instead, this commit generates x, y at
random in [-10,10], which should better corresponds to real applications.
Reviewed-by: Florian Weimer <fweimer@redhat.com>