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"

Start by locating the CPython standard-library test module covering os, file descriptors, pipes, and TemporaryFile, then run the existing filesystem tests under Emscripten. Add the three proposed tests where related filesystem behavior is tested; done means they are included in the standard-library suite and pass without regressing other tests.