tests: run multiple pytests in parallel in the workflow #48

Job	Run time
Unittest (test.mod, py36)	43s
Unittest (test.mod, py39)	49s
Unittest (test.mod, py311)	50s
Unittest (test.mod, py312)	54s
Unittest (test.run.test_assemblers, py36)	44s
Unittest (test.run.test_assemblers, py39)	57s
Unittest (test.run.test_assemblers, py311)	40s
Unittest (test.run.test_assemblers, py312)	1m 12s
Unittest (test.run.test_boot, py36)	52s
Unittest (test.run.test_boot, py39)	59s
Unittest (test.run.test_boot, py311)	1m 4s
Unittest (test.run.test_boot, py312)	51s
Unittest (test.run.test_devices, py36)	1m 8s
Unittest (test.run.test_devices, py39)	39s
Unittest (test.run.test_devices, py311)	54s
Unittest (test.run.test_devices, py312)	45s
Unittest (test.run.test_executable, py36)	52s
Unittest (test.run.test_executable, py39)	56s
Unittest (test.run.test_executable, py311)	55s
Unittest (test.run.test_executable, py312)	1m 1s
Unittest (test.run.test_mount, py36)	48s
Unittest (test.run.test_mount, py39)	40s
Unittest (test.run.test_mount, py311)	40s
Unittest (test.run.test_mount, py312)	45s
Unittest (test.run.test_noop, py36)	53s
Unittest (test.run.test_noop, py39)	55s
Unittest (test.run.test_noop, py311)	40s
Unittest (test.run.test_noop, py312)	42s
Unittest (test.run.test_sources, py36)	1m 5s
Unittest (test.run.test_sources, py39)	54s
Unittest (test.run.test_sources, py311)	47s
Unittest (test.run.test_sources, py312)	42s
Unittest (test.run.test_stages, py36)	59s
Unittest (test.run.test_stages, py39)	50s
Unittest (test.run.test_stages, py311)	51s
Unittest (test.run.test_stages, py312)	56s
	30m 52s

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