Limit max execution time for `test-linux-stable` CI jobs (#3483)

Tests run as part of the `test-linux-stable` jobs could hang as shown
[here](https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5341393).
This PR adds a failsafe for such cases.

.gitlab/pipeline/test.yml

@@ -48,6 +48,7 @@ test-linux-stable:
       - target/nextest/default/junit.xml
     reports:
       junit: target/nextest/default/junit.xml
+  timeout: 90m
 
 test-linux-oldkernel-stable:
   extends: test-linux-stable