NFC: Fix gmock integration in unit tests (#8879)

Tested with the following code which passes (in arena.cpp, not in
source-map.cpp which would have already worked previously due to how it
was handled in the build file):


```c++
#include "gmock/gmock-matchers.h"
...
EXPECT_THAT(1, ::testing::Eq(1));
```

This was written by Gemini. I didn't quite understand the fix but I
figured it's worth doing to make tests easier to write. Below is
Gemini's description of the fix.

---

Previously, when `BUILD_FUZZTEST=ON`, the `fuzztest` dependency would
download its own newer version of `googletest` (v1.14.0) via CMake
`FetchContent`. The `gmock` headers bundled in Binaryen's
`third_party/googletest/googlemock` were incompatible with this newer
`googletest`. This caused compilation failures for tests using `gmock`.
As a workaround, `source-map.cpp` was completely excluded from the test
suite during fuzztest builds.

This fix works because `fuzztest`'s `FetchContent` actually makes the
`gmock` target available as well. By linking `binaryen-unittests`
against the `gmock` target when `BUILD_FUZZTEST=ON`, CMake correctly
configures the include paths to use the newly fetched `gmock` headers
instead of the bundled ones, seamlessly resolving the incompatibility.

- Unconditionally include `source-map.cpp` in unit tests, rather than
conditionally disabling it when `BUILD_FUZZTEST` is ON.
- Link against the `gmock` target for `binaryen-unittests` when
`BUILD_FUZZTEST` is ON.