Add custom working directory value to run tests

[rlalik]

Small update allowing to change the location (relative to the tested repository) where pytest should start to look for tests.

I had a case where I wanted to have separate graders for subprojects, like my tree structure:

.
├── zadanie_1
│   └── tests
├── zadanie_2
│   └── tests
└── zadanie_3
    └── tests

and I wanted to run grader in each subdirectory separately:

    steps:
    - name: Checkout code
      uses: actions/checkout@v4
    - name: Python Test - Zadanie 1
      id: test-zadanie-1
      uses: rlalik/autograding-python-grader@v1.1
      with:
        timeout: 10
        max-score: 1.5
        working-directory: zadanie_1
    - name: Python Test - Zadanie 2
      id: test-zadanie-2
      uses: rlalik/autograding-python-grader@v1.1
      with:
        timeout: 10
        max-score: 2
        setup-command: pip install -r requirements.txt
        working-directory: zadanie_2
    - name: Python Test - Zadanie 3
      id: test-zadanie-3
      uses: rlalik/autograding-python-grader@v1.1
      with:
        timeout: 10
        max-score: 1.5
        working-directory: zadanie_3
    - name: Autograding Reporter
      uses: classroom-resources/autograding-grading-reporter@v1
      env:
        TEST-ZADANIE-1_RESULTS: "${{steps.test-zadanie-1.outputs.result}}"
        TEST-ZADANIE-2_RESULTS: "${{steps.test-zadanie-2.outputs.result}}"
        TEST-ZADANIE-3_RESULTS: "${{steps.test-zadanie-3.outputs.result}}"
      with:
        runners: test-zadanie-1,test-zadanie-2,test-zadanie-3

giving following summary:

| Test runner summary
| ┌────────────────────┬─────────────┬─────────────┐
| │ Test Runner Name   │ Test Score  │ Max Score   │
| ├────────────────────┼─────────────┼─────────────┤
| │ test-zadanie-1     │ 1.5         │ 1.5         │
| ├────────────────────┼─────────────┼─────────────┤
| │ test-zadanie-2     │ 1.2         │ 2           │
| ├────────────────────┼─────────────┼─────────────┤
| │ test-zadanie-3     │ 1.5         │ 1.5         │
| ├────────────────────┼─────────────┼─────────────┤
| │ Total:             │ 4.2         │ 5           │
| └────────────────────┴─────────────┴─────────────┘
| 🏆 Grand total tests passed: 15/16