Missing handling for HTTP 503 from Icinga 2 API in integration tests

[julianbrost]

Currently, the integration tests are failing quite frequently due to the Icinga 2 API returning HTTP 503 responses:

    sla_test.go:237: 
        	Error Trace:	sla_test.go:237
        	            				sla_test.go:367
        	Error:      	Not equal: 
        	            	expected: 200
        	            	actual  : 503
        	Test:       	TestSla/DowntimeEvents/Fixed/Expire
        	Messages:   	schedule-downtime

Examples for pipelines with this or similar errors on GitHub Actions: 1 2 3 4

This is probably a result of Icinga/icinga2#9445, so the tests should be adapted to handle this.

[yhabteab]

I think this is a bug and a blocker, as otherwise all other PRs suffer from this issue.

[Al2Klimov]

TODO

• fix our dep github.com/icinga/icinga-testing (in its upstream) Icinga2Client#Do(): call parent method, re-try on HTTP 503 300 times icinga-testing#29
• bump its "version" here

[yhabteab]

[x] bump its "version" here

Done #696!