[compiler-rt][Profile] Wait for child threads in set-file-object test

We've been seeing this test return 31 instead of 32 for the "functions"
line in this test on our AArch64 bots.

One possible cause is some of the children not finishing in time
before the llvm-profdata commands are run, if the machine is heavily loaded.

Wait for all the children to finish before exiting the parent.

Reviewed By: zequanwu

Differential Revision: https://reviews.llvm.org/D109222