Fork test fails when perf timers are enabled

[stevenengler]

When shadow is built with ./setup build --test --debug --use-perf-timers, the fork test (and other tests that use fork) fail:

The following tests FAILED:
	 15 - fork-shadow (Failed)
	 52 - bash-example-shadow (Failed)

thread 'shadow-worker' panicked at main/utility/perf_timer.rs:22:9:
assertion failed: self.start_time.is_none()
stack backtrace:
   0: rust_begin_unwind
             at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/std/src/panicking.rs:595:5
   1: core::panicking::panic_fmt
             at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/core/src/panicking.rs:67:14
   2: core::panicking::panic
             at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/core/src/panicking.rs:117:5
   3: shadow_rs::utility::perf_timer::PerfTimer::start
             at /shared/code/shadow/rust-syscall-wrappers/src/main/utility/perf_timer.rs:22:9
   4: shadow_rs::host::process::RunnableProcess::start_cpu_delay_timer
             at /shared/code/shadow/rust-syscall-wrappers/src/main/host/process.rs:462:9
   5: shadow_rs::host::process::Process::start_cpu_delay_timer
             at /shared/code/shadow/rust-syscall-wrappers/src/main/host/process.rs:1153:9
   6: shadow_rs::host::process::Process::resume
             at /shared/code/shadow/rust-syscall-wrappers/src/main/host/process.rs:1190:9
   7: shadow_rs::host::host::Host::resume
             at /shared/code/shadow/rust-syscall-wrappers/src/main/host/host.rs:482:13
   8: shadow_rs::core::work::task::TaskRef::execute
             at /shared/code/shadow/rust-syscall-wrappers/src/main/core/work/task.rs:32:9