refactor: introduce and use InputOrStaticDefault (#632)

This commit introduces a new `InputLoader` policy by which, if no
input is provided, we use a static default input list.

We also modify the code to use this policy for dnscheck and
stunreachability, with proper input.

We also modify `miniooni` to pass the new `ExperimentName` field to
the `InputLoader` to indicate which default input list to use.

This diff is part of a set of diffs aiming at fixing
https://github.com/ooni/probe/issues/1814 and has been
extracted from https://github.com/ooni/probe-cli/pull/539.

What remains to be done, after this diff has landed is to ensure
things also work for ooniprobe and oonimkall.

internal/engine/allexperiments.go

@@ -49,7 +49,7 @@ var experimentsByName = map[string]func(*Session) *ExperimentBuilder{
 				))
 			},
 			config:      &dnscheck.Config{},
-			inputPolicy: InputStrictlyRequired,
+			inputPolicy: InputOrStaticDefault,
 		}
 	},
 
@@ -198,7 +198,7 @@ var experimentsByName = map[string]func(*Session) *ExperimentBuilder{
 				))
 			},
 			config:      &stunreachability.Config{},
-			inputPolicy: InputStrictlyRequired,
+			inputPolicy: InputOrStaticDefault,
 		}
 	},