check: implement `check.Context.HasValue`

[GregoryAlbouy]

Description

Current check.ContextChecker implementation requires to use a ValueChecker for equality check on a context value:

check.Context.Value(someContextKey, check.Wrap(check.String.Is("some value")))
// or
check.Context.Value(someContextKey, check.Value.Is("some value"))

We suspect equality checks to be rather common, thus the following would be convenient:

check.Context.HasValue(someContextKey, "some value")

Todo

• Implement checkContextCheckerProvider.HasValue