Fix Lint/DuplicateMethods in StringIO_test.rb

[soutaro]

Summary

master's CI ("Ruby" job) is failing on RuboCop with Lint/DuplicateMethods:

Lint/DuplicateMethods: Method `StringIOTypeTest#test_pread` is defined at both test/stdlib/StringIO_test.rb:78 and test/stdlib/StringIO_test.rb:133.
Lint/DuplicateMethods: Method `StringIOTypeTest#test_read_nonblock` is defined at both test/stdlib/StringIO_test.rb:83 and test/stdlib/StringIO_test.rb:140.
Lint/DuplicateMethods: Method `StringIOTypeTest#test_sysread` is defined at both test/stdlib/StringIO_test.rb:93 and test/stdlib/StringIO_test.rb:162.

Cause

This is a merge skew (semantic conflict) between two PRs that were each green on their own:

• test_pread / test_read_nonblock / test_sysread (the full-argument variants) were already on master via commit 4de713c ("Add stdlib signature tests for Correct core/stdlib signatures to match Ruby 4.0 #2967").
• Allow nil output buffers for reader methods #3002 ("Allow nil output buffers for reader methods") added methods of the same name (the nil output buffer variants) in commit 1914368.

#3002's branch last synced master at a point that did not yet contain 4de713c, so RuboCop saw no duplicate when #3002's CI ran. The duplicate only materialized once #3002 was merged into the current master, and CI is not re-run against the post-merge state.

Fix

Merge the nil output buffer assertions into the existing methods and drop the duplicate definitions. No test coverage is lost.

• ruby -c: Syntax OK
• No duplicate method names remain in the file.

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