Export symbols from `snappy_test_support` when building shared with MSVC

When attempting to build a shared library with MSVC:

```
cmake .. -G "Ninja Multi-Config" -DCMAKE_BUILD_SHARED:BOOL=ON
```

The following error occurs:

```
[36/48] Linking CXX executable Release\snappy_test_tool.exe
FAILED: Release/snappy_test_tool.exe
cmd.exe /C "cd . && C:\Tools\cmake-3.25.2-windows-x86_64\bin\cmake.exe -E vs_link_exe --intdir=CMakeFiles\snappy_test_tool.dir\Release --rc=C:\PROGRA~2\WI3CF2~1\10\bin\100226~1.0\x64\rc.exe --mt=C:\PROGRA~2\WI3CF2~1\10\bin\100226~1.0\x64\mt.exe --manifests  -- C:\PROGRA~1\MIB055~1\2022\LTSC-17.8\PROFES~1\VC\Tools\MSVC\1438~1.331\bin\Hostx64\x64\link.exe /nologo CMakeFiles\snappy_test_tool.dir\Release\snappy_test_tool.cc.obj  /out:Release\snappy_test_tool.exe /implib:Release\snappy_test_tool.lib /pdb:Release\snappy_test_tool.pdb /version:0.0 /machine:x64 /INCREMENTAL:NO /subsystem:console  Release\snappy_test_support.lib  Release\snappy.lib  kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cd ."
LINK: command "C:\PROGRA~1\MIB055~1\2022\LTSC-17.8\PROFES~1\VC\Tools\MSVC\1438~1.331\bin\Hostx64\x64\link.exe /nologo CMakeFiles\snappy_test_tool.dir\Release\snappy_test_tool.cc.obj /out:Release\snappy_test_tool.exe /implib:Release\snappy_test_tool.lib /pdb:Release\snappy_test_tool.pdb /version:0.0 /machine:x64 /INCREMENTAL:NO /subsystem:console Release\snappy_test_support.lib Release\snappy.lib kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTFILE:Release\snappy_test_tool.exe.manifest" failed (exit code 1181) with the following output:
LINK : fatal error LNK1181: cannot open input file 'Release\snappy_test_support.lib'
```

This appears to be because no symbols are exported from the `snappy_test_support` library, and so a `.lib` file is not created. This works correctly for the `snappy` library, so the same logic should be applied to the `snappy_test_support` library.

[]