Some proposed stdlib filesystem tests

The following three tests are failing on Emscripten main because of my faulty dup implementation. @tiran didn't notice and fix it, and indeed the Python stdlib tests don't seem to notice that dup isn't working. I would be happy to make a PR adding these to the standard library tests given advice on where to put them.

def test_dup_pipe():
    # See https://github.com/emscripten-core/emscripten/issues/14640
    import os

    [fdr1, fdw1] = os.pipe()
    fdr2 = os.dup(fdr1)
    fdw2 = os.dup2(fdw1, 50)
    # Closing any of fdr, fdr2, fdw, or fdw2 will currently destroy the pipe.
    # This bug is fixed upstream:
    # https://github.com/emscripten-core/emscripten/pull/14685
    s1 = b"some stuff"
    s2 = b"other stuff to write"
    os.write(fdw1, s1)
    assert os.read(fdr2, 100) == s1
    os.write(fdw2, s2)
    assert os.read(fdr1, 100) == s2

def test_dup_temp_file():
    # See https://github.com/emscripten-core/emscripten/issues/15012
    import os
    from tempfile import TemporaryFile

    tf = TemporaryFile(buffering=0)
    fd1 = os.dup(tf.fileno())
    s = b"hello there!"
    tf.write(s)
    tf2 = open(fd1, "w+")
    assert tf2.tell() == len(s)
    # This next assertion actually demonstrates a bug in dup: the correct value
    # to return should be b"".
    assert os.read(fd1, 50) == b""
    tf2.seek(1)
    assert tf.tell() == 1
    assert tf.read(100) == b"ello there!"

def test_dup_stdout():
    # Test redirecting stdout using low level os.dup operations.
    # This sort of redirection is used in pytest.
    import os
    import sys
    from tempfile import TemporaryFile

    tf = TemporaryFile(buffering=0)
    save_stdout = os.dup(sys.stdout.fileno())
    os.dup2(tf.fileno(), sys.stdout.fileno())
    print("hi!!")
    print("there...")
    assert tf.tell() == len("hi!!\nthere...\n")
    os.dup2(save_stdout, sys.stdout.fileno())
    print("not captured")
    os.dup2(tf.fileno(), sys.stdout.fileno())
    print("captured")
    assert tf.tell() == len("hi!!\nthere...\ncaptured\n")
    os.dup2(save_stdout, sys.stdout.fileno())
    os.close(save_stdout)
    tf.seek(0)
    assert tf.read(1000).decode() == "hi!!\nthere...\ncaptured\n"

Beginne damit, das CPython-Testmodul der Standardbibliothek zu finden, das os, Dateideskriptoren, Pipes und TemporaryFile abdeckt, und führe dann die vorhandenen Dateisystemtests unter Emscripten aus. Füge die drei vorgeschlagenen Tests dort hinzu, wo das entsprechende Dateisystemverhalten getestet wird; als erledigt gilt die Aufgabe, wenn sie in der Standardbibliothekstestsuite enthalten sind und bestehen, ohne dass andere Tests regressieren.