Bug: `utDefaultIOStream.FileSizeTest` test fails since a79dc358cfa9b065e1fb2183247956a254cd79dd

**Describe the bug**
`utDefaultIOStream.FileSizeTest` test fails since a79dc358cfa9b065e1fb2183247956a254cd79dd.

```
[ RUN ] utDefaultIOStream.FileSizeTest
/build/source/test/unit/UnitTestFileGenerator.h:88: Failure
Expected: (-1) != (fd), actual: -1 vs -1

/build/source/test/unit/utDefaultIOStream.cpp:67: Failure
Expected: (nullptr) != (fs), actual: (nullptr) vs NULL

[ FAILED ] utDefaultIOStream.FileSizeTest (0 ms)
```

**Platform (please complete the following information):**
- OS: `linux`

**Additional context**
Add any other context about the problem here.

I think a79dc358cfa9b065e1fb2183247956a254cd79dd changed `MakeTmpFile()` semantics from changing template in-place to never modify it. And that broke the test.

Start with test/unit/UnitTestFileGenerator.h and test/unit/utDefaultIOStream.cpp, focusing on MakeTmpFile() and the failing FileSizeTest. Compare their expectations with commit a79dc358cfa9b065e1fb2183247956a254cd79dd, then run the Linux unit test to confirm the failure. Done means utDefaultIOStream.FileSizeTest passes without regressing temporary-file behavior.

Written by the indexing model from the issue text.