refactor(tests): split integration.rs into focused subfolders

- Delete 20 duplicate tests already covered by parameter_tests.rs
- Move 11 pure Trial unit tests into src/trial.rs
- Split remaining 84 integration tests into tests/study/ (9 modules)
- Group sampler tests into tests/sampler/ (7 modules)
- Group pruner tests into tests/pruner/ (2 modules)

tests/sampler/main.rs

@@ -0,0 +1,15 @@
+#![allow(
+    clippy::cast_sign_loss,
+    clippy::cast_precision_loss,
+    clippy::cast_possible_truncation
+)]
+
+mod bohb;
+#[cfg(feature = "cma-es")]
+mod cma_es;
+mod differential_evolution;
+#[cfg(feature = "gp")]
+mod gp;
+mod multivariate_tpe;
+mod random;
+mod tpe;