config: platforms-chromeos: Add serial delay for some Mediatek platforms

Add test_character_delay to the Spherion, Tomato and Steelix platforms
to workaround the fact that they're sometimes unable to process serial
input fast enough, resulting in mangled commands and consequently flaky
test results, as described in
kernelci/kernelci-project#366.

The right place to do this change would be in the device-type template
as described in LAVA's documentation [1]. This overriding in KernelCI is
meant only as a temporary workaround to verify whether this fixes the
issue. If it does, then we'll do it in LAVA upstream instead.

[1] https://docs.lavasoftware.org/lava/debugging.html#differences-in-input-speeds
Signed-off-by: Nícolas F. R. A. Prado <[email protected]>

config/platforms-chromeos.yaml

@@ -118,6 +118,8 @@ platforms:
       flash_kernel:
         url: https://storage.chromeos.kernelci.org/images/rootfs/chromeos/chromiumos-corsola/20240514.0/arm64
         image: 'Image'
+    context:
+      test_character_delay: 10
     rules:
       <<: *arm64-chromebook-device-rules
       min_version:
@@ -128,6 +130,8 @@ platforms:
     <<: *mediatek-chromebook-device
     base_name: asurada
     dtb: dtbs/mediatek/mt8192-asurada-spherion-r0.dtb
+    context:
+      test_character_delay: 10
     rules:
       <<: *arm64-chromebook-device-rules
       min_version:
@@ -138,6 +142,8 @@ platforms:
     <<: *mediatek-chromebook-device
     base_name: cherry
     dtb: dtbs/mediatek/mt8195-cherry-tomato-r2.dtb
+    context:
+      test_character_delay: 10
     rules:
       <<: *arm64-chromebook-device-rules
       min_version: