Previously, calling IO#flush or closing an IO with unflushed buffered
writes would just invoke `#blocking_operation_wait` and flush the write
buffer using a `write` syscall. This change adds flushing through the
fiber scheduler by invoking the `#io_write` hook.

* Prefer IO::Buffer#write in IOScheduler

* Use Dir.tmpdir for test file

* Correctly handle errors in io_flush_buffer_fiber_scheduler

In the child process, nullify the current fiber scheduler and set the current fiber to blocking.

```
test/fiber/test_scheduler.rb:98: warning: Scheduler should implement #fiber_interrupt
```

Previously, vm_make_env_each() (used during proc
creation and for the debug inspector C API) picked up the
non-GC-allocated iseq that rb_vm_push_frame_fname() creates,
which led to a SEGV when the GC tried to mark the non GC object.

Put a real iseq imemo instead. Speed should be about the same since
the old code also did a imemo allocation and a malloc allocation.

Real iseq allows ironing out the special-casing of dummy frames in
rb_execution_context_mark() and rb_execution_context_update(). A check
is added to RubyVM::ISeq#eval, though, to stop attempts to run dummy
iseqs.

[Bug #21180]

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

[Bug #20907]

`ppc64le` appears to be struggling with this test due to timeout. Let's see
if reducing the number of iterations can help improve the test performance.