tests: allow mount tests to run in parallel #51

Job	Run time
Unittest (test.mod, py36)	59s
Unittest (test.mod, py39)	57s
Unittest (test.mod, py311)	1m 4s
Unittest (test.mod, py312)	57s
Unittest (test.run.test_assemblers, py36)	3m 14s
Unittest (test.run.test_assemblers, py39)	3m 14s
Unittest (test.run.test_assemblers, py311)	3m 13s
Unittest (test.run.test_assemblers, py312)	3m 17s
Unittest (test.run.test_boot, py36)	3m 15s
Unittest (test.run.test_boot, py39)	3m 14s
Unittest (test.run.test_boot, py311)	3m 14s
Unittest (test.run.test_boot, py312)	3m 13s
Unittest (test.run.test_devices, py36)	1m 8s
Unittest (test.run.test_devices, py39)	49s
Unittest (test.run.test_devices, py311)	41s
Unittest (test.run.test_devices, py312)	43s
Unittest (test.run.test_executable, py36)	53s
Unittest (test.run.test_executable, py39)	44s
Unittest (test.run.test_executable, py311)	49s
Unittest (test.run.test_executable, py312)	45s
Unittest (test.run.test_mount, py36)	1m 15s
Unittest (test.run.test_mount, py39)	1m 3s
Unittest (test.run.test_mount, py311)	1m 1s
Unittest (test.run.test_mount, py312)	44s
Unittest (test.run.test_noop, py36)	56s
Unittest (test.run.test_noop, py39)	59s
Unittest (test.run.test_noop, py311)	58s
Unittest (test.run.test_noop, py312)	1m 8s
Unittest (test.run.test_sources, py36)	49s
Unittest (test.run.test_sources, py39)	44s
Unittest (test.run.test_sources, py311)	42s
Unittest (test.run.test_sources, py312)	45s
Unittest (test.run.test_stages, py36)	1m 1s
Unittest (test.run.test_stages, py39)	57s
Unittest (test.run.test_stages, py311)	58s
Unittest (test.run.test_stages, py312)	1m 23s
	51m 46s

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