Stabilize regression test result.
If random() returns a result sufficiently close to zero, float8out
switches to scientific notation, breaking this test case's expectation
that the output should look like '0.xxxxxxxxx'. Casting to numeric
should fix that. Per buildfarm member pogona.
Discussion: https://postgr.es/m/
20180324212502.wt4serghfidge2on@alap3.anarazel.de