Fix observer tests on Windows

Use %e instead of a hardcoded forward slash.

Disable opcache optimizations during some observer tests

Opcache inlines functions that only return a constant. Disable
optimizations to prevent differences in tests where such functions

Disable opcache optimizations during some observer tests

Opcache inlines functions that only return a constant. Disable
optimizations to prevent differences in tests where such functions
are used (or rewrite the test to not depend on it).

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Provide unused retvals to observers

Make sure that the return value is available to observers, even if
it is not used by the caller.

Closes GH-6422.