ci: add tests workflow (#120)

For now, the tests fail. At least the `nimble build` step of the new
workflow succeeds in CI - it didn't on Linux until the previous
commit [1].

Add only Linux x86_64 for now. On macOS x86_64, the workflow would
error with:

    clang: error: no such file or directory: '/Users/runner/.local/c0/libs/libffi.a'

and on macOS arm64 it would error with:

    ERROR: unable to find archive matching pattern macosx_arm64.tar.xz

because the Nim nightlies repo doesn't have a binary build for that
platform yet.

[1] 2024-04-08, 5e7a21c, "fix: make nimble build succeed on Linux"

.github/workflows/tests.yml

@@ -0,0 +1,46 @@
+name: tests
+
+on:
+  push:
+    branches:
+      - dev
+      - jtv/v2
+      - main
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  tests:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: linux
+            runs-on: ubuntu-22.04
+            arch: x86_64
+
+    name: "${{ matrix.os }}-${{ matrix.arch }}"
+    runs-on: ${{ matrix.runs-on }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@0ad4b8fadaa221de15dcec353f45205ec38ea70b # 4.1.4
+
+      - name: Install Nim
+        uses: iffy/install-nim@c76b9157e544576bf61c932ee66ff3c7d194deb9 # 5.0.4
+        with:
+          version: "binary:2.0.0"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} # Avoid rate limiting.
+
+      - name: Install Nimble dependencies
+        run: nimble --accept install --depsOnly
+
+      - name: Build
+        run: nimble build
+
+      - name: Run tests
+        run: ./test