build: use matrix for split unit tests #111

Job	Run time
filter	1m 0s
build(8) for showcase	9m 11s
build(8) except for gapic-generator-java	4m 0s
build(8) for gapic-generator-java	11m 13s
build(25) except self-service clients	11m 26s
showcase-clirr	3m 29s
dependency_analyzer_unit_test	1m 12s
build (11)	13m 35s
build (17)	13m 1s
build(21) except self-service clients	12m 43s
showcase (11)	9m 35s
showcase (25)	9m 41s
showcase (17)	9m 59s
showcase (21)	9m 40s
gapic-generator-java-bom	2m 11s
unmanaged_dependency_check	9m 57s
	2h 11m 53s

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