build: use matrix for split unit tests #16857

Job	Run time
owlbot-py	1m 8s
root-pom	1m 49s
gapic-bom	1m 55s
gitignore_correctness	1m 6s
package_name_check	59s
generate-readme	1m 12s
owlbot-yaml	1m 6s
group_id_check_for_maps_libraries	7m 38s
	16m 53s

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