chore: Add a test workflow

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: pip
+    directory: "/.github/workflows"
+    schedule:
+      interval: daily
+      time: "12:00"
+      timezone: "UTC"
+    reviewers: []
+    labels: []

.github/workflows/constraints.txt

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+pip==22.1.1
+poetry==1.1.13
+virtualenv==20.14.1

.github/workflows/test.yml

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+name: Test
+
+on:
+  pull_request: {}
+  push:
+    branches: [main]
+  workflow_dispatch:
+    inputs: {}
+
+jobs:
+  tests:
+    name: Test on ${{ matrix.python-version }} / ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    continue-on-error: false
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+        - { python-version: "3.7", os: "ubuntu-latest" }
+        - { python-version: "3.8", os: "ubuntu-latest" }
+        - { python-version: "3.9", os: "ubuntu-latest" }
+        - { python-version: "3.10", os: "ubuntu-latest" }
+
+    steps:
+    - name: Check out the repository
+      uses: actions/[email protected]
+
+    - name: Install Poetry
+      env:
+        PIP_CONSTRAINT: .github/workflows/constraints.txt
+      run: |
+        pipx install poetry
+        poetry --version
+
+    - name: Setup Python ${{ matrix.python-version }}
+      uses: actions/[email protected]
+      with:
+        python-version: ${{ matrix.python-version }}
+        architecture: x64
+        cache: 'poetry'
+
+    - name: Upgrade pip
+      env:
+        PIP_CONSTRAINT: .github/workflows/constraints.txt
+      run: |
+        pip install pip
+        pip --version
+
+    - name: Install Dependencies
+      run: |
+        poetry env use "${{ matrix.python-version }}"
+        poetry install
+
+    # TODO: Run complete test suite. Requires setting up some repository secrets
+    - name: Run tests
+      run: |
+        poetry run pytest --doctest-modules singer_sdk
+        poetry run pytest tests/core
+        poetry run pytest tests/cookiecutters