FileIO, BufferedWriter left unflushed when ipy exits

• Running the following code with ipy, will get an empty file test.txt, while with CPython, the file will have one byte 'a'

import io
out = io.FileIO('test.txt', 'w')
out.write(b'a')

• Similar case but using BufferedWriter:

import io
out = io.TextIOWrapper(io.BufferedWriter(io.FileIO('test.txt', 'w')))
out.write('a')

• Case involving stdio:

The following powershell command output nothing,

PS C:\Users\james\proj\ironpython3> .\bin\Debug\netcoreapp3.1\ipy.exe -c "import sys;sys.stdout.write('text')"
PS C:\Users\james\proj\ironpython3>

and the following powershell command could output the "text"

PS C:\Users\james\proj\ironpython3> .\bin\Debug\netcoreapp3.1\ipy.exe -c "import sys;sys.stdout.write('text\n')"
text
PS C:\Users\james\proj\ironpython3>

Reproduce the FileIO, BufferedWriter/TextIOWrapper, and stdout examples with ipy and compare their behavior with CPython. Trace the process-exit handling for these streams; done means pending file and standard-output data is emitted when ipy exits, including data without a trailing newline.