ci: increase subprocess suite parallelism (#15224)

## Description

Saw a timeout, trying to increase parallelism to improve chances of
success.

## Testing

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## Risks

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-->

## Additional Notes

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Author: brettlangdon

tests/contrib/suitespec.yml

@@ -1225,7 +1225,7 @@ suites:
     runner: riot
     snapshot: true
   subprocess:
-    parallelism: 1
+    parallelism: 2
     paths:
       - '@bootstrap'
       - '@core'