Update flutter_devtools.test (#315)

This updates the DevTools to the current latest hash from
https://github.com/flutter/devtools/commits/master.

It includes flutter/devtools#6824 which relands
some changes to help users always use DevTools pinned version of
Flutter, but now in a way that allows this script to still use its own
(head) version of Flutter.

registry/flutter_devtools.test

@@ -3,7 +3,7 @@
 [email protected]
 
 fetch=git clone https://github.com/flutter/devtools.git tests
-fetch=git -C tests checkout 79cde662349a2c92a24c79ab3983a05569d4b436
+fetch=git -C tests checkout 3e7a39eb2576fe7029a8475f35c9af82641adddf
 
 setup.linux=./tool/flutter_customer_tests/setup.sh >> output.txt