Disable incompatible tests on Linux AArch64

[mhalbritter]

When running the Spring Boot build with ./gradlew build, the EmbeddedMongoAutoConfigurationTests fail all the time. This is because Embedded Mongo has no support for Linux aarch64 (See flapdoodle-oss/de.flapdoodle.embed.mongo#379).

We should workaround this by disabling the tests if running on Linux aarch64.

[mhalbritter]

org.springframework.boot.web.embedded.jetty.SslServerCustomizerTests#whenHttp2IsEnabledServerConnectorsHasSslAlpnH2AndHttpConnectionFactories and org.springframework.boot.web.embedded.jetty.SslServerCustomizerTests#alpnConnectionFactoryHasNullDefaultProtocolToAllowNegotiationToHttp11 also don't work on Linux aarch64, see google/conscrypt#1051

They are now ignored on linux aarch64, too.

[wilkinsona]

@sbrannen Do you think it would be worth proposing an enhancement to JUnit that provides a built-in condition for enabling/disabling tests on particular OS architectures?