ci: add secp256 unit test workflow

This workflow job detects whether the secp256k1 subtree was updated,
and if it was will build and run the secp256k1 unit tests.

These are disabled by default (in other CI jobs), as they double the
runtime needed, and rarely change.

Author: willcl-ark

.github/workflows/secp256.yml

@@ -0,0 +1,37 @@
+# Copyright (c) 2025-present The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit.
+
+name: secp256k1 tests
+on:
+  pull_request:
+    paths:
+      - 'src/secp256k1/**'
+  push:
+    paths:
+      - 'src/secp256k1/**'
+
+env:
+  MAKEJOBS: '-j10'
+
+jobs:
+  secp256k1-tests:
+    name: 'secp256k1 tests'
+    runs-on: ubuntu-24.04
+    if: github.event_name == 'pull_request'
+    timeout-minutes: 120
+    steps:
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y build-essential cmake pkgconf python3 libevent-dev libboost-dev libsqlite3-dev
+      - name: Build with secp256k1 tests enabled
+        run: |
+          cmake -B build \
+            -DSECP256K1_BUILD_TESTS=ON \
+            -DSECP256K1_BUILD_EXHAUSTIVE_TESTS=ON
+          cmake --build build -j $(nproc)
+      - name: Run secp256k1 tests
+        run: |
+          # Run only the secp256k1 tests
+          ctest --test-dir build --output-on-failure -R "secp256k1" -j $(nproc)