[test] refactor common test target settings (#27013)
### Description
- factor duplicated test target settings into helper functions
- reuse helpers for onnxruntime_test_all and onnxruntime_provider_test
- keep target-specific settings intact
### Motivation and Context
There are some duplicated codes in the onnxruntime_unittests. Originally
there is only one unit test `onnxruntime_test_all` and later it is split
into two: `onnxruntime_test_all` and `onnxruntime_provider_test`. Some
lines for setting up build flags are simply copied. This causes
potential risk for inconsistency in future.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
