gh-63161: Add more tests for source encoding

[serhiy-storchaka]

• Issue: Non-UTF8 encoding line #63161

[vstinner]

Two tests are failing:

======================================================================
FAIL: test_nul_in_first_coding_line (test.test_source_encoding.FileSourceEncodingTest.test_nul_in_first_coding_line)
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Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_source_encoding.py", line 316, in test_nul_in_first_coding_line
    self.check_script_error(src, br"source code (string )?cannot contain null bytes")
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_source_encoding.py", line 425, in check_script_error
    self.assertRegex(res.err.rstrip().splitlines()[-1], b'SyntaxError: ' + expected)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Regex didn't match: b'SyntaxError: source code (string )?cannot contain null bytes' not found in b'\x1b[1;35mSyntaxError\x1b[0m: \x1b[35msource code cannot contain null bytes\x1b[0m'

======================================================================
FAIL: test_nul_in_second_coding_line (test.test_source_encoding.FileSourceEncodingTest.test_nul_in_second_coding_line)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_source_encoding.py", line 323, in test_nul_in_second_coding_line
    self.check_script_error(src, br"source code (string )?cannot contain null bytes")
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_source_encoding.py", line 425, in check_script_error
    self.assertRegex(res.err.rstrip().splitlines()[-1], b'SyntaxError: ' + expected)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Regex didn't match: b'SyntaxError: source code (string )?cannot contain null bytes' not found in b'\x1b[1;35mSyntaxError\x1b[0m: \x1b[35msource code cannot contain null bytes\x1b[0m'

[miss-islington-app]

Thanks @serhiy-storchaka for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-139442 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-139443 is a backport of this pull request to the 3.13 branch.