Rewrite the 'exported extension method' tests; add basic extension tests #4091

Job	Run time
test (ubuntu-latest, dev, main)	5m 3s
test (ubuntu-latest, dev, flutter)	20m 10s
test (ubuntu-latest, dev, packages)	1m 14s
test (ubuntu-latest, dev, sdk-docs)	1m 17s
test (ubuntu-latest, stable, main)	4m 57s
test (ubuntu-latest, stable, packages)	2m 15s
test (macos-latest, dev, main)	7m 10s
test (windows-latest, dev, main)	4m 29s
	46m 35s

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