Report could not find hits as test case error

[mrodm]

Currently, "could not find hits in <data_stream> data stream" error is not reported as part of the xUnit report.

For instance, in this build https://buildkite.com/elastic/integrations/builds/8409#018d4368-0d0c-4ddd-a1b9-1f6c3d4b6753 , no hits were found and elastic-package returned an error without completing the test run. Moreover, there is no jUnit report (XML file)
because of that failure:

Error: error running package system tests: could not complete test run: could not find hits in logs-aws.vpcflow-ep data stream

https://buildkite.com/elastic/integrations/builds/8409#018d4368-0d0c-4ddd-a1b9-1f6c3d4b6753/6-10235

This PR adds this specific error as a ErrTestCaseFailed so it gets added into the system xUnit report. As a side-effect, elastic-package now allows to run the other tests even if for one data stream there are no hits found.

Report example for mysql package:

<?xml version="1.0" encoding="UTF-8"?>
<testsuites>
  <testsuite name="system" tests="10" failures="2">
    <!--test suite for system tests-->
    <testcase name="system test: default (variant: mysql_8_0_35)" classname="mysql.slowlog" time="38.373606196"></testcase>
    <testcase name="system test: default (variant: percona_8_0_36)" classname="mysql.slowlog" time="39.211554004"></testcase>
    <testcase name="system test: default (variant: mysql_8_0_35)" classname="mysql.status" time="41.172481079"></testcase>
    <testcase name="system test: default (variant: percona_8_0_36)" classname="mysql.status" time="41.041845086"></testcase>
    <testcase name="system test: default (variant: mysql_8_0_35)" classname="mysql.error" time="639.829253568">
      <failure>could not find hits in logs-mysql.error-ep data stream</failure>
    </testcase>
    <testcase name="system test: default (variant: percona_8_0_36)" classname="mysql.error" time="636.446832929">
      <failure>could not find hits in logs-mysql.error-ep data stream</failure>
    </testcase>
    <testcase name="system test: default (variant: mysql_8_0_35)" classname="mysql.galera_status" time="39.484423072"></testcase>
    <testcase name="system test: default (variant: percona_8_0_36)" classname="mysql.galera_status" time="40.702275199"></testcase>
    <testcase name="system test: default (variant: mysql_8_0_35)" classname="mysql.performance" time="39.012759805"></testcase>
    <testcase name="system test: default (variant: percona_8_0_36)" classname="mysql.performance" time="38.714038382"></testcase>
  </testsuite>
</testsuites>

How to test locally

elastic-package stack up -v -d

# Go to `mysql` package in integrations repository
cd /path/integrations/repo/packages/mysql

elastic-package test system -v --report-format xUnit --report-output file

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 4c68f72

cc @mrodm