tests: increase CI integration test parallelism

Increase the number of parallel integration tests in CI to save ~3-5
minutes per x86_64 run. The thread limit is driven by RAM and disk
space constraints, not CPU availability.

A new `PARALLEL_INTEGRATION_TESTS_NUM` environment variable controls
the thread count. In CI it is set explicitly (12 for x86_64, 25 for
ARM64); locally it falls back to `nproc / 4`, preserving the previous
behavior.

Only the first test group (`common_parallel`, `live_migration_parallel`)
uses the overridden value - subsequent groups (dbus_api, fw_cfg,
ivshmem, aarch64_acpi) continue to use the `nproc / 4` default.

On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

.github/workflows/integration-arm64.yaml

@@ -7,6 +7,11 @@ concurrency:
 jobs:
   build:
     timeout-minutes: 120
+    env:
+      # Our runner has 80 cores (nproc).
+      # We limit parallelism only to avoid exhausting disk space and memory
+      # resources, not to save CPU resources.
+      PARALLEL_INTEGRATION_TESTS_NUM: 25
     name: Tests (ARM64)
     runs-on: ${{ github.event_name == 'pull_request' && 'ubuntu-latest' || 'bookworm-arm64' }}
     steps:

.github/workflows/integration-x86-64.yaml

@@ -7,6 +7,11 @@ concurrency:
 jobs:
   build:
     timeout-minutes: 80
+    env:
+      # Our runner has 16 cores (nproc).
+      # We limit parallelism only to avoid exhausting disk space and memory
+      # resources, not to save CPU resources.
+      PARALLEL_INTEGRATION_TESTS_NUM: 12
     strategy:
       fail-fast: false
       matrix: