build: use matrix for split unit tests #105

Job	Run time
filter	1m 4s
build(25) except self-service clients	12m 25s
build(21) except self-service clients	12m 48s
build(8) for showcase	9m 3s
build(8) except for gapic-generator-java	3m 48s
build(8) for gapic-generator-java	11m 1s
build (11)	13m 35s
gapic-generator-java-bom	1m 57s
dependency_analyzer_unit_test	1m 15s
showcase-clirr	3m 32s
unmanaged_dependency_check	8m 56s
build (17)	13m 48s
showcase (25)	9m 56s
showcase (21)	10m 14s
showcase (17)	10m 2s
showcase (11)	10m 20s
	2h 13m 44s

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