Keep the Plugin Tests' Python Packages Out of the Working Directory

The plugin test fixtures start the interpreter before the test points
data_dir at its temporary directory, so PythonInterpreter creates
{data_dir}/python/packages and {data_dir}/log with an empty data_dir: a
python/ and log/ folder in whatever directory the tests run from. When that
is the test binary's folder, the next run's embedded-interpreter tests took
the stray python/ as their home and failed to start Python.

Give each fixture that initializes the plugin manager its own temporary
data directory, set up before initialize(), and only use the python/ folder
next to the test binary as the interpreter's home when it holds a standard
library.

tests/slic3rutils/test_plugin_cloud_metadata.cpp

@@ -31,6 +31,9 @@ namespace {
 // same as any other plugin.
 struct ScopedManagerShutdown
 {
+    // Before initialize(): the interpreter creates {data_dir}/python/packages and {data_dir}/log,
+    // which would otherwise land in the working directory.
+    ScopedDataDir python_data_dir{"plugin-python"};
     bool initialized = PluginManager::instance().initialize();
 
     ~ScopedManagerShutdown()