SecuritySettingsSource license.self_generated: trial (#38233)

Authn is enabled only if `license_type` is non `basic`, but `basic` is
what the `LicenseService` generates implicitly. This commit explicitly sets
license type to `trial`, which allows for authn, in the `SecuritySettingsSource`
which is the settings configuration parameter for `InternalTestCluster`s.

The real problem, that had created tests failures like #31028 and #32685, is
that the check `licenseState.isAuthAllowed()` can change sporadically. If it were
to return `true` or `false` during the whole test there would be no problem.
The problem manifests when it turns from `true` to `false` right before `Realms.asList()`.
There are other license checks before this one (request filter, token service, etc)
that would not cause a problem if they would suddenly see the check as `false`.
But switching to `false` before `Realms.asList()` makes it appear that no installed
realms could have handled the authn token which is an authentication error, as can
be seen in the failing tests.

Closes #31028 #32685

x-pack/plugin/security/src/test/java/org/elasticsearch/test/SecuritySettingsSource.java

@@ -137,7 +137,8 @@ public Settings nodeSettings(int nodeOrdinal) {
                 .put(LoggingAuditTrail.EMIT_NODE_NAME_SETTING.getKey(), randomBoolean())
                 .put(LoggingAuditTrail.EMIT_NODE_ID_SETTING.getKey(), randomBoolean())
                 .put("xpack.security.authc.realms." + FileRealmSettings.TYPE + ".file.order", 0)
-                .put("xpack.security.authc.realms." + NativeRealmSettings.TYPE + ".index.order", "1");
+                .put("xpack.security.authc.realms." + NativeRealmSettings.TYPE + ".index.order", "1")
+                .put("xpack.license.self_generated.type", "trial");
         addNodeSSLSettings(builder);
         return builder.build();
     }