test_asn1.rb: Remove the assertions of the time string format without second.

[junaruga]

This PR fixes #724.

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This commit fixes the following errors in the tests. Because the OpenSSL project changed the code to make the time string format without second invalid. So, we drop the assertions.

1) Error: test_generalizedtime(OpenSSL::TestASN1): OpenSSL::ASN1::ASN1Error: generalizedtime is too short
/home/runner/work/ruby-openssl/ruby-openssl/test/openssl/test_asn1.rb:698:in `decode'
/home/runner/work/ruby-openssl/ruby-openssl/test/openssl/test_asn1.rb:698:in `decode_test'
/home/runner/work/ruby-openssl/ruby-openssl/test/openssl/test_asn1.rb:433:in `test_generalizedtime'
     430:       OpenSSL::ASN1::GeneralizedTime.new(Time.utc(9999, 9, 8, 23, 43, 39))
     431:     # LibreSSL 3.6.0 requires the seconds element
     432:     return if libressl?
  => 433:     decode_test B(%w{ 18 0D }) + "201612081934Z".b,
     434:       OpenSSL::ASN1::GeneralizedTime.new(Time.utc(2016, 12, 8, 19, 34, 0))
     435:     # not implemented
     436:     # decode_test B(%w{ 18 13 }) + "20161208193439+0930".b,

2) Error: test_utctime(OpenSSL::TestASN1): OpenSSL::ASN1::ASN1Error: utctime is too short
/home/runner/work/ruby-openssl/ruby-openssl/test/openssl/test_asn1.rb:698:in `decode'
/home/runner/work/ruby-openssl/ruby-openssl/test/openssl/test_asn1.rb:698:in `decode_test'
/home/runner/work/ruby-openssl/ruby-openssl/test/openssl/test_asn1.rb:411:in `test_utctime'
     408:     end
     409:     # Seconds is omitted. LibreSSL 3.6.0 requires it
     410:     return if libressl?
  => 411:     decode_test B(%w{ 17 0B }) + "1609082343Z".b,
     412:       OpenSSL::ASN1::UTCTime.new(Time.utc(2016, 9, 8, 23, 43, 0))
     413:     # not implemented
     414:     # decode_test B(%w{ 17 11 }) + "500908234339+0930".b,