Add integration tests for xDS flow control #2428

Job	Run time
set-execution-conditions	12s
build agent and cache	1m 53s
download midwares and cache	5s
Test for spring (1.5.0.RELEASE, Edgware.SR2, 1.5.0.RELEASE)	15m 32s
Test for spring (2.0.2.RELEASE, Finchley.RELEASE, 4.1.3, 2.0.0.RELEASE, 1.0.1)	16m 7s
Test for spring (2.1.0.RELEASE, Greenwich.RELEASE, 4.2.6, 2.1.0.RELEASE, 1.0.1)	15m 48s
Test for spring (2.2.0.RELEASE, Hoxton.RELEASE, 4.3.6, 2.2.0.RELEASE, 1.0.3)	15m 5s
Test for spring (2.3.0.RELEASE, Hoxton.RELEASE, 4.4.1, 2.2.8.RELEASE, 1.1.2)	15m 23s
Test for spring (2.4.0, 2020.0.0, 4.5.6, 2021.0.1.0, 1.1.4)	15m 6s
Test for spring (2.6.2, 2021.0.0, 4.5.10, 2021.1, 1.1.10)	14m 37s
Test for spring (2.7.2, 2021.0.3, 4.5.13, 1.1.12)	14m 17s
	2h 4m 5s

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