Migrate FlBasicMessageChannel tests to FlMockBinaryMessenger (#57115)

Use FlMockBinaryMessenger instead of mocking the whole engine.
https://dart.googlesource.com/external/github.com/flutter/engine/+/c5962d12fd81482641d2cc022994ee1e512a1aff

DEPS

diff --git a/DEPS b/DEPS
index 37c8c00..99ff6db 100644
--- a/DEPS
+++ b/DEPS
@@ -256,7 +256,7 @@
 deps = {
   'engine/src': 'https://github.com/flutter/buildroot.git' + '@' + 'cc9bcddf1524812c80ef741191d5db7469e705de',
 
-  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'f100e640ebb9d613c906bdac1a3361ae897acc10',
+  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'c5962d12fd81482641d2cc022994ee1e512a1aff',
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '83e6b860adb06bba87e1e3591f7ed616d4d846a7',
 

commits.json

diff --git a/commits.json b/commits.json
index 96e82a6..a464a6d 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "engine/src/flutter":"f100e640ebb9d613c906bdac1a3361ae897acc10",
+  "engine/src/flutter":"c5962d12fd81482641d2cc022994ee1e512a1aff",
   "engine/src/flutter/third_party/dart":"83e6b860adb06bba87e1e3591f7ed616d4d846a7",
   "flutter":"6966a2eef1e98fc326ea240525f1d42e0b4a4806"
 }
\ No newline at end of file