refactor: replace context.Background() with t.Context()/b.Context() in tests (#7352)

Based on the Go version we currently use, the dependency already
supports 1.24+, which allows using `t.Context()` and `b.Context()` in
unit tests and benchmarks respectively.

- Enable `context-background` and `context-todo` in
[`usetesting`](https://golangci-lint.run/docs/linters/configuration/#usetesting)
- Adjust the code to support linter detection

---------

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Tyler Yahn <codingalias@gmail.com>
Co-authored-by: Damien Mathieu <42@dmathieu.com>

internal/global/alternate_meter_test.go

@@ -267,7 +267,7 @@ func TestMeterDelegation(t *testing.T) {
 
 	SetMeterProvider(amp)
 
-	ctx := context.Background()
+	ctx := t.Context()
 	ao := &altObserver{t: t}
 	for _, meter := range amp.meters {
 		for _, cb := range meter.cbs {