server/.../backoff,syncs: don't break builds without synctest (#9484)

Previously, developers without the synctest experiment enabled would see
build failures when running tests in some server/internal/internal
packages using the synctest package. This change makes the transition to
use of the package less painful but guards the use of the synctest
package with build tags.

synctest is enabled in CI. If a new change will break a synctest
package, it will break in CI, even if it does not break locally.

The developer docs have been updated to help with any confusion about
why package tests pass locally but fail in CI.

server/internal/internal/backoff/backoff_test.go

@@ -1,3 +1,5 @@
+//go:build goexperiment.synctest
+
 package backoff
 
 import (

server/internal/internal/syncs/line_test.go

@@ -1,3 +1,5 @@
+//go:build goexperiment.synctest
+
 package syncs
 
 import (