Fix pgbench permute tests.

One of the tests for the pgbench permute() function added by
6b258e3d68 fails on some 32-bit platforms, due to variations in the
floating point computations in getrand(). The remaining tests give
sufficient coverage, so just remove the failing test.

Reported by Christoph Berg. Analysis by Thomas Munro and Tom Lane.
Based on patch by Fabien Coelho.

Discussion: https://postgr.es/m/YKQnUoYV63GRJBDD@msg.df7cb.de
