test: do not open fixture files for writing
test-fs-fsync makes a copy of a fixture file, but then doesn't do anything with it and instead operates on the original fixture file. This appears to be in error, and this change fixes that.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
test