pw_string: Add EXPECT calls for static analyzer
pw_string: Add EXPECT calls for static analyzer

In addition to the existing static asserts, explicitly check the value
of test values using the EXPECT macros.

Change-Id: Ifad02d8058b1d7f60535cd8cf92c1d5bb5b2fa86
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/31972
Pigweed-Auto-Submit: Prashanth Swaminathan <7074f112da8ec89069105f3b18a4ce46a4722488@google.com>
Commit-Queue: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>

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