Disable TLS diagnostics in FIPS mode (#49900)

This commit sets xpack.security.ssl.diagnose.trust to false in all
the nodes of our TestClusters when running integTest. This is needed
in 7.x because setting xpack.security.ssl.diagnose.trust to true
wraps SunJSSE TrustManager with our own DiagnosticTrustManager and
this is not allowed when SunJSSE is in FIPS mode.
An alternative would be to set `xpack.security.fips.enabled` to
true which would also implicitly disable
xpack.security.ssl.diagnose.trust but would have additional effects
(would require that we set PBKDF2 for password hashing algorithm in
all test clusters, would prohibit using JKS keystores in nodes even
if relevant tests have been muted in FIPS mode etc.)

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -160,6 +160,8 @@ class BuildPlugin implements Plugin<Project> {
                             testClusters.all { ElasticsearchCluster cluster ->
                                 cluster.systemProperty 'javax.net.ssl.trustStorePassword', 'password'
                                 cluster.systemProperty 'javax.net.ssl.keyStorePassword', 'password'
+                                // Can't use our DiagnosticTrustManager with SunJSSE in FIPS mode
+                                cluster.setting 'xpack.security.ssl.diagnose.trust', 'false'
                             }
                         }
                     }