Fix failing tests (#3947)

Changes:
- fix scheduler tests: Unrequesting can cause an underflow if root
unnoted the preimage in the meantime.
This should normally not happen - but can - and there is a test for it.
 - fix core-fellowship tests that also seem broken :(
 - merge in #3885

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Javier Viola <363911+pepoviola@users.noreply.github.com>
Co-authored-by: Serban Iorga <serban@parity.io>
Co-authored-by: Adrian Catangiu <adrian@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>
Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>
Co-authored-by: Dastan <88332432+dastansam@users.noreply.github.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Clara van Staden <claravanstaden64@gmail.com>
Co-authored-by: Ron <yrong1997@gmail.com>
Co-authored-by: Vincent Geddes <vincent@snowfork.com>
Co-authored-by: Svyatoslav Nikolsky <svyatonik@gmail.com>
Co-authored-by: Bastian Köcher <info@kchr.de>

.gitlab/pipeline/zombienet.yml

@@ -1,7 +1,8 @@
 .zombienet-refs:
   extends: .build-refs
   variables:
-    ZOMBIENET_IMAGE: "docker.io/paritytech/zombienet:v1.3.98"
+    ZOMBIENET_IMAGE: "docker.io/paritytech/zombienet:v1.3.99"
+    PUSHGATEWAY_URL: "http://zombienet-prometheus-pushgateway.managed-monitoring:9091/metrics/job/zombie-metrics"
 
 include:
   # substrate tests