WildFly TS intergration/manualmode Health tests are failing intermittently because of an unexpected HTTP 404 response

We noticed this happening on WildFly while testing EAP7-1763 and related changes [1].
The feature hasn't been ported to the EAP XP 4 branch, yet, so there's no EAP affected version, nor a target release.
These fields can be adjusted later on.

As per the title, the issue affects only those tests in the integration/manualmode [2] module and is intermittent.
The goal of this JIRA is to track the effort needed to determine whether it is a server issue or not.

ATM we are testing a fix which is based on the assumption that this is a test case issue - also because this issue was not seen during pre-checking and doesn't happen in EAP QE CI - meaning that the tests logic should be modified in order to wait for the subsystem to be started. It is a quick-fix for the alleged root cause.

If it turns out to be true, then this JIRA will be updated to track the work needed in order to fix the TS in future, with lower priority.

Otherwise priority and description can be changed and also affect the EAP7-1763 status but we are assuming that this should not be the case, ATM.

See https://issues.redhat.com/browse/EAP7-1763?focusedCommentId=18816123&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-18816123

Stack trace exception fragment for future reference:

Caused by: java.lang.RuntimeException: org.wildfly.test.manual.microprofile.health.InvalidHealthResponseException: Expected UP but received HTTP status code 404
	at org.wildfly.test.manual.microprofile.health.MicroProfileHealthDefaultEmptyStartupTestBase.lambda$testApplicationStartupWithoutDeploymentWithEmptyStartupSetToUP$3(MicroProfileHealthDefaultEmptyStartupTestBase.java:268)
	at java.base/java.util.concurrent.CompletableFuture...

CC mstefank

[1]
https://github.com/wildfly/wildfly/pull/14400

[2]
https://github.com/xstefank/wildfly/tree/WFLY-14940/testsuite/integration/manualmode/src/test/java/org/wildfly/test/manual/microprofile/health