gh-138013: Split TextIO tests from test_io.test_general

[cmaloney]

These tests take 1.3 seconds on my dev machine, match fairly closely with testing textio.c implementation only.

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I'm only planning one more split to these tests after this, finally moving the buffered out of test_general. Timings with -uall,largefile,extralargefile,walltime -M12G

- test_io.test_signals: 24.2 sec
- test_io.test_largefile: 10.0 sec
- test_io.test_general: 8.9 sec
- test_io.test_textio: 1.3 sec
- test_io.test_fileio: 375 ms
- test_io.test_bufferedio: 293 ms
- test_io.test_file: 87 ms
- test_io.test_memoryio: 56 ms
- test_io.test_univnewlines: 31 ms

• Issue: Move Buffered tests from test_io to test_bufio #138013

[cmaloney]

cc: @vstinner (only one more to go after this than test_io split will be done)

[vstinner]

Merged, thank you.