PY-88280 Create venv in .venv subdirectory in pyVenvFixture

Previously the fixture created the venv directly as the module content
root. Since inner virtualenvs are excluded from the module source roots,
this caused the entire content root to be excluded. Create the venv in
a .venv subdirectory instead, matching real-world project layout.

(cherry picked from commit 492e2cfa9cdc01af91610f8579d771dd3a8eaa2d)

GitOrigin-RevId: 6a5f5491e46b39c28089e73bd15f5e6f8af2d192

python/python-test-env/junit5/src/com/intellij/python/test/env/junit5/pyVenvFixture.kt

@@ -31,7 +31,7 @@ fun TestFixture<SdkFixture<PyEnvironment>>.pyVenvFixture(
   val env = this@pyVenvFixture.init().env
   withContext(Dispatchers.EDT) {
     val module = moduleFixture?.init()
-    val venvDir = where.init()
+    val venvDir = where.init().resolve(".venv")
     val venvPython = createVenv(env.pythonPath, venvDir).getOrThrow()
     val venvSdk = withContext(Dispatchers.IO) { createSdk(venvPython) }
     if (addToSdkTable) {