build: use matrix for split unit tests #136

Job	Run time
filter	1m 1s
build(8) except for gapic-generator-java	4m 27s
build(8) for showcase	9m 29s
showcase-clirr	4m 52s
build(25) except self-service clients	12m 33s
build (11)	14m 6s
showcase (11)	9m 59s
unmanaged_dependency_check	9m 37s
build(8) for gapic-generator-java	11m 8s
build (17)	13m 57s
dependency_analyzer_unit_test	1m 17s
showcase (17)	9m 51s
showcase (25)	10m 3s
showcase (21)	9m 58s
build(21) except self-service clients	12m 46s
gapic-generator-java-bom	3m 4s
	2h 18m 8s

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