Refactor test scripts.

Move helper classes to test_helper folder. Only test case files
in the first level.

Test: atest -v CtsSeekerDiscoverProviderTest
BUG: 214015364
Change-Id: I088adcb647d01ac558ce7dd1fea880821d59028e
diff --git a/nearby/tests/multidevices/host/Android.bp b/nearby/tests/multidevices/host/Android.bp
index b0adfba..bad8a26 100644
--- a/nearby/tests/multidevices/host/Android.bp
+++ b/nearby/tests/multidevices/host/Android.bp
@@ -37,6 +37,5 @@
 
 python_library_host {
     name: "NearbyMultiDevicesHostHelper",
-    srcs: ["*.py"],
-    exclude_srcs: ["*_test.py"],
+    srcs: ["test_helper/*.py"],
 }
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