Cover the native esp-idf toolchain in the BW and 4C test fixtures

PlatformIO and the native (non-PlatformIO) esp-idf toolchain build
components differently - only the latter caught the missing-public-
include bug fixed in 12b1119. Pin one BW and one color model test to
toolchain: esp-idf for regression coverage of that path.

tests/test_gxepd2_420.esp32-idf.yaml

@@ -3,6 +3,7 @@ esphome:
 
 esp32:
   board: esp32dev
+  toolchain: esp-idf
   framework:
     type: esp-idf
 

tests/test_gxepd2_4c.esp32-idf.yaml

@@ -2,6 +2,7 @@ esphome:
   name: gxepd2-test-4c
 esp32:
   board: esp32dev
+  toolchain: esp-idf
   framework:
     type: esp-idf
 external_components: