[tests] Use larger app partition for esp32-c6-idf component builds

The default IDF partition leaves only 1.75MB for the app which is too small for
grouped tests (bluetooth_proxy currently overflows). Match esp32-idf, esp32-c2-idf,
esp32-c3-idf and esp32-s3-idf by switching the c6 base config to partitions_testing.csv.

tests/test_build_components/build_components_base.esp32-c6-idf.yaml

@@ -6,6 +6,9 @@ esp32:
   board: esp32-c6-devkitc-1
   framework:
     type: esp-idf
+  # Use custom partition table with larger app partition (3MB)
+  # Default IDF partitions only allow 1.75MB which is too small for grouped tests
+  partitions: ../partitions_testing.csv
 
 logger:
   level: VERY_VERBOSE