fix: Rework telemetry tests to use synctest (#1631)

Replace uses of a `sync.WaitGroup` to coordinate between different
goroutines in tests with `testing/synctest`. That package, stable
starting with Go 1.25, simplifies testing concurrent and asynchronous
code by providing:

- *Fake time*: Time advances automatically when goroutines block.
- *Quiescence detection*: Deterministic synchronization instead of
  arbitrary delays.
- *Fast, reliable tests*: Because of the fake time mechanism, test can
  wait for "arbitrary" periods of time and they will work. The clock
  will advance, but wallclock will be short. At the same time, you don't
  have to guess how much is enough.

Signed-off-by: Marcelo E. Magallon <marcelo.magallon@grafana.com>

Author: mem

go.mod

@@ -1,8 +1,8 @@
 module github.com/grafana/synthetic-monitoring-agent
 
-go 1.24.0
+go 1.25.0
 
-toolchain go1.24.6
+toolchain go1.25.4
 
 require (
 	github.com/go-kit/kit v0.13.0