build: add config to build Electron for tests (#14101)

* build: add config to build Electron for tests

* build: run nightly linux builds for the 3-0-x branch

* build: gn: update CI config for linux builds

- run GN debug and testing builds for PRs (as FYI)
- do not run tests for GN debug builds
- run GN release builds nightly

* build: gn: run testing builds on CI instead of release on Windows

Should save some time since release builds set 'official_build=true'
which make the builds take much more time.

* build: gn: use testing config to run CI jobs on Mac

Tests should run faster for testing builds.

build/args/testing.gn

@@ -0,0 +1,7 @@
+import("all.gn")
+is_debug = false
+is_component_build = false
+is_official_build = false
+dcheck_always_on = true
+symbol_level = 1
+use_jumbo_build = true