Add concurrent test support #5845

Job	Run time
Build Ballerina Lang	5m 29s
Build Ballerina Distribution	41m 47s
Build Stdlib Level (2)	11m 29s
Build Stdlib Level (3)	5m 50s
Build Stdlib Level (1)	4m 37s
Build Stdlib Level (4)	15m 8s
Build Stdlib Level (5)	34m 55s
Build Stdlib Level (6)	20m 3s
Build Stdlib Level (8)	34m 2s
Build Stdlib Level (9)	5m 53s
Build Stdlib Level (7)	19m 43s
	3h 18m 56s

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