Add a design doc for distributed execution and test suites #3018

Job	Run time
test-prev (1.20.x, ubuntu-latest)	3m 23s
test-current-cov (1.21.x, ubuntu-latest)	8m 6s
test-prev (1.20.x, windows-2019)	9m 41s
test-current-cov (1.21.x, windows-2019)	12m 45s
test-tip (ubuntu-latest)	4m 31s
test-prev (1.20.x, macos-latest)	8m 4s
test-current-cov (1.21.x, macos-latest)	13m 7s
test-tip (windows-2019)	10m 22s
test-tip (macos-latest)	13m 28s
	1h 23m 27s

Provide feedback