Rework fetch_bytes and _fetch_pypi_json patches to make testing easier. Various improvements to test_micropip. The main feature is a new fixture
'mock_fetch' with an add_pkg method that takes a package name, a map
from version to requirements, and a choice of platform. This should
hopefully make writing more tests a lot easier (which is good because
we could use more micropip test coverage but we are limited by the
difficulty of writing good tests).

This also adds a fixture to create distinct dummy package names and
enables @pytest.mark.asyncio to handle the async calls rather than
using `asyncio.get_event_loop.run_until_complete`.
from pyodide_test_runner import run_in_pyodide


@run_in_pyodide(packages=["cffi"])
def test_cffi_asprintf():
    from cffi import FFI

    ffi = FFI()
    ffi.cdef(
        """int asprintf(char** buf, const char *format, ...);   // copy-pasted from the man page"""
    )
    C = ffi.dlopen(None)  # loads the entire C namespace
    buf = ffi.new("char**")
    arg1 = ffi.new("char[]", b"wo")
    arg2 = ffi.new("char[]", b"ld")
    C.asprintf(buf, b"hello %sr%s", arg1, arg2)
    assert ffi.string(buf[0]).decode() == "hello world"

from pyodide_test_runner import run_in_pyodide


@run_in_pyodide(packages=["cffi"])
def test_cffi_asprintf():
    from cffi import FFI

    ffi = FFI()
    ffi.cdef(
        """int asprintf(char** buf, const char *format, ...);   // copy-pasted from the man page"""
    )
    C = ffi.dlopen(None)  # loads the entire C namespace
    buf = ffi.new("char**")
    arg1 = ffi.new("char[]", b"wo")
    arg2 = ffi.new("char[]", b"ld")
    C.asprintf(buf, b"hello %sr%s", arg1, arg2)
    assert ffi.string(buf[0]).decode() == "hello world"
