Keep the Plugin Tests' Python Packages Out of the Working Directory (#15981)

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_lifecycle.cpp

@@ -42,6 +42,9 @@ namespace {
 // Declare this FIRST in a test so it is destroyed last.
 struct ScopedPluginManager
 {
+    // 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 = false;
 
     ScopedPluginManager() { initialized = PluginManager::instance().initialize(); }