Fix test runner to use current Python and report failures

[xdong99]

Summary

• Run test scripts with sys.executable so the active virtualenv/CI Python is preserved.
• Resolve test files relative to the tests/ directory, allowing python tests/runtests.py from the repo root.
• Exit non-zero when any test script fails.
• Replace os.system("python -m docxtpl ...") with subprocess.run(..., check=True).

Verification

• python tests/runtests.py
• python -m flake8 docxtpl tests setup.py --count --max-line-length=127 --show-source --statistics

[arturict]

Good docs-oriented PR. Could you include a short negative test case for the new placeholder support to ensure error behavior remains stable?

[JackSpiece]

Checked this locally from the repository root with the project environment. All 36 test scripts completed and the runner returned 0. I also reran it with the project import deliberately unavailable; it collected the failing script names and returned 1 as intended. The module CLI test used the same interpreter and generated its output successfully. The full flake8 target passes too. No blocking issues found.