Add parallel test target, using pytest-xdist #815

Job	Run time
Windows ARM64	16m 39s
Windows AMD64	41m 37s
Windows x86	22m 24s
manylinux_2_28 x86_64	25m 40s
macOS 10.13 x86_64	21m 34s
macOS 10.15 x86_64	19m 20s
manylinux_2_28 aarch64	24m 5s
macOS arm64	23m 16s
manylinux2014 and musllinux aarch64	38m 48s
macOS 10.10 x86_64	18m 47s
manylinux2014 and musllinux x86_64	43m 43s
sdist	26s
Upload release to PyPI	0s
Upload wheels to scientific-python-nightly-wheels	0s
	4h 56m 19s

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