Fix sporadic bug in the write_ptrace test

[nbaksalyar]

It looks like a race condition that hasn't been observed in local environments yet.
The error occurs in the write_memory_ptrace test:

---- target::linux::writemem::tests::write_memory_ptrace stdout ----

thread 'target::linux::writemem::tests::write_memory_ptrace' panicked at 'called `Result::unwrap()` on an `Err` value: Sys(EPERM)', src/target/linux/writemem.rs:292:91

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:

    target::linux::writemem::tests::write_memory_ptrace

A possible strategy to find the root case:

1. Replicate the Travis CI environment (same Docker env settings and capabilities).
2. Set a breakpoint on the if let Err(Sys(EPERM)) = result { ... } condition.
3. Run the test repeatedly until the breakpoint is hit, and observe the local state.