testing: runtest.py improvements ported from OpenImageIO

[lgritz]

Port the parts of OIIO PR 5465 and 5476 that apply to OSL's runtest.py:

• run_app() takes a failureok parameter. If the first word of the command names an OSL app (oslc, oslinfo, testshade, testrender, testoptix) or an OIIO app (oiiotool, maketx, etc.), it is replaced with the full path to the built app.
• New run_commands() takes a multi-line string of shell commands, one per line, and runs each through run_app(). Blank lines and # comments are skipped. So a run.py can list commands exactly as they would be typed, and docs can include them by reference.
• The oslc, oslinfo, oiiotool, maketx, oiiodiff, testshade, testrender, and testoptix helpers are now thin wrappers around run_app(). oiiotool() also gained failureok and concat parameters.
• text_diff ignores trailing whitespace differences, which can vary by platform (e.g. cmd.exe's echo).
• redirect_push and redirect_pop to let a test redirect a portion of output to a different file.

Assisted-by: Claude Code / Claude Opus 5.5