`exit(0)` was causing memory errors during shutdown of this forked process, in an unreliable way (only visible on CI system).
Example: https://build.bitcoinabc.org/viewLog.html?buildId=28627
@Fabien and I found that GCC-compiled versions of this test were failing with --logger=JUNIT passed in to the boost test suite. The most reliable way to see the errors on exit (with GCC-compiled) was:
valgrind ./src/test/test_bitcoin --logger=JUNIT -t util_tests
Since we're using `fork()` anyway, we might as well just run `execlp()` which nukes the child process image. As a bonus this stops valgrind from producing any noise about this child process.