[Support] unflake TempFileCollisions test

Summary:
This test was added to verify that createUniqueEntity() does
not enter an infinite loop when all possible names are taken. However,
it also checked that all possible names are generated, which is flaky
(because the names are generated randomly). This change increases the
number of attempts we make to make flakes exceedingly
unlikely (3.88e-62).

Reviewers: fedor.sergeev, rsmith

Reviewed By: fedor.sergeev

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D56336

llvm-svn: 358914