Move trailing-comma config tests to check tests

[YingqiDuan]

Refs #5966.

This is a test-only change that moves multiline trailing-comma coverage from three subprocess-based cmdline.test cases into existing check-flags.test cases:

• testCmdlineCfgEnableErrorCodeTrailingComma -> testPerModuleErrorCodesOverride
• testCmdlineCfgDisableErrorCodeTrailingComma -> testDisableErrorCodeConfigFile
• testCmdlineCfgAlwaysTrueTrailingComma -> testAlwaysTrueAlwaysFalseConfigFile

The existing check cases exercise the configured behavior, so the command-line cases can be removed without losing coverage. In testPerModuleErrorCodesOverride, truthy-bool is placed last so the value immediately before the trailing comma is exercised by the test.

Tests:

• python -m pytest -n0 -q mypy/test/testcheck.py::TypeCheckSuite::check-flags.test (219 passed)
• python -m pytest -n0 -q mypy/test/testcmdline.py -k "testCmdlineCfgFilesTrailingComma or testCmdlineCfgModulesTrailingComma or testCmdlineCfgPackagesTrailingComma" (3 passed)

Local targeted timing on Windows with pytest parallelism disabled (-n0), averaged across three runs:

• Before, as three subprocess-based cmdline.test cases: 9.08s
• After, as three check-flags.test cases: 0.54s