build: use matrix for split unit tests #103

Job	Run time
filter	58s
build(8) for showcase	9m 33s
build(25) except self-service clients	12m 31s
build(8) except for gapic-generator-java	4m 49s
gapic-generator-java-bom	3m 7s
showcase-clirr	3m 25s
build(8) for gapic-generator-java	12m 15s
build (11)	14m 3s
build(21) except self-service clients	12m 53s
showcase (25)	10m 21s
build (17)	14m 31s
showcase (17)	10m 17s
dependency_analyzer_unit_test	1m 14s
showcase (21)	10m 7s
showcase (11)	9m 55s
unmanaged_dependency_check	9m 24s
	2h 19m 23s

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