Fix TimeoutError on TruffleRuby

[andrykonchin]

The issue was reported here truffleruby/truffleruby#2800.

It happens that all the test cases in tests/openssl/test_engine.rb fail with error Timeout::Error: execution of assert_separately expired on TruffleRuby head.

Timeout is caused by recursive requiring and auto-running the tests/openssl/test_engine.rb file.

openssl/test/openssl/test_engine.rb

Lines 75 to 81 in 4f1267d

	def with_openssl(code, **opts)
	assert_separately([{ "OSSL_MDEBUG" => nil }, "-ropenssl"], <<~"end;", **opts)
	require #{__FILE__.dump}
	include OpenSSL::TestEngine::Utils
	#{code}
	end;
	end

Looks like the purpose of this require is making available in the forked process some helpers defined in the test file.

The fix repairs a mechanism to disable auto-running tests in a forked process. Test::Unit::Runner isn't available and looks like is a part of CRuby's own test infrastructure (it's defined here).

[hsbt]

I submit another fixes for this at #593