ci: add ubuntu-latest test pipelines for highest/lowest torch

Add 4 GitHub-hosted Linux pipelines that run on ubuntu-latest:
- highest-torch slow, highest-torch fast
- lowest-torch slow, lowest-torch fast

Each runs the corresponding make target with --junit and uploads its
test-results artifact. Ungated and independent of the macOS self-hosted
jobs so they also run on forks.

Author: guru-desh

.github/workflows/ci.yaml

@@ -58,3 +58,83 @@ jobs:
         with:
           name: test-results-macos-highest-fast
           path: test-results/
+
+  linux-highest-slow:
+    name: Linux / make test-highest-pytorch / markers=slow
+    runs-on: ubuntu-latest
+    timeout-minutes: 120
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - name: Install uv
+        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+        with:
+          enable-cache: false
+      - name: Run `make test-highest-pytorch`
+        run: make test-highest-pytorch PYTEST_ARGS="--marker slow --junit"
+      - name: Upload test results
+        if: always()
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: test-results-linux-highest-slow
+          path: test-results/
+
+  linux-highest-fast:
+    name: Linux / make test-highest-pytorch / markers=not-slow
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - name: Install uv
+        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+        with:
+          enable-cache: false
+      - name: Run `make test-highest-pytorch`
+        run: make test-highest-pytorch PYTEST_ARGS="--marker 'not slow' --junit"
+      - name: Upload test results
+        if: always()
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: test-results-linux-highest-fast
+          path: test-results/
+
+  linux-lowest-slow:
+    name: Linux / make test-lowest-pytorch / markers=slow
+    runs-on: ubuntu-latest
+    timeout-minutes: 120
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - name: Install uv
+        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+        with:
+          enable-cache: false
+      - name: Run `make test-lowest-pytorch`
+        run: make test-lowest-pytorch PYTEST_ARGS="--marker slow --junit"
+      - name: Upload test results
+        if: always()
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: test-results-linux-lowest-slow
+          path: test-results/
+
+  linux-lowest-fast:
+    name: Linux / make test-lowest-pytorch / markers=not-slow
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - name: Install uv
+        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+        with:
+          enable-cache: false
+      - name: Run `make test-lowest-pytorch`
+        run: make test-lowest-pytorch PYTEST_ARGS="--marker 'not slow' --junit"
+      - name: Upload test results
+        if: always()
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: test-results-linux-lowest-fast
+          path: test-results/