fix(test): allow rack 2/3 via RACK_VERSION to fix x509 tests #178

Job	Run time
test-with-active-support (2.7, ubuntu-latest)	26s
test (2.7, ubuntu-latest)	51s
test-with-active-support (2.7, windows-latest)	2m 6s
test-with-rack-2 (3.2, ubuntu-latest)	8s
test (2.7, windows-latest)	3m 19s
test-with-active-support (2.7, macos-latest)	28s
test-with-rack-2 (3.2, windows-latest)	1m 55s
test (2.7, macos-latest)	42s
test-with-active-support (3.0, ubuntu-latest)	7s
test-with-rack-2 (3.2, macos-latest)	13s
test (3.0, ubuntu-latest)	38s
test-with-active-support (3.0, windows-latest)	1m 39s
test-with-active-support (3.0, macos-latest)	11s
test (3.0, windows-latest)	3m 14s
test-with-active-support (3.1, ubuntu-latest)	8s
test (3.0, macos-latest)	28s
test-with-active-support (3.1, windows-latest)	1m 3s
test (3.1, ubuntu-latest)	40s
test-with-active-support (3.1, macos-latest)	14s
test (3.1, windows-latest)	2m 42s
test-with-active-support (3.2, ubuntu-latest)	7s
test (3.1, macos-latest)	40s
test-with-active-support (3.2, windows-latest)	1m 18s
test (3.2, ubuntu-latest)	37s
test-with-active-support (3.2, macos-latest)	12s
test (3.2, windows-latest)	3m 58s
test (3.2, macos-latest)	27s
test (3.3, ubuntu-latest)	42s
test (3.3, windows-latest)	3m 28s
test (3.3, macos-latest)	35s
	33m 16s

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