s1024: don't suggest an invalid fix when Sub is not called on a time.Time - #1730

Fixes #756.

S1024 rewrites x.Sub(time.Now()) into time.Until(x), which only compiles when x is itself a time.Time. Sub is reachable in two other shapes, and the fix produces code that does not build in either:

type embedTime struct{ time.Time }

func fnEmbedded(t embedTime) { t.Sub(time.Now()) } // fix: time.Until(t)
func fnPointer(t *time.Time) { t.Sub(time.Now()) } // fix: time.Until(t)

The pointer case is not in the issue, but it has the same cause.

Emitting time.Until(t.Time) for the embedded case would be nicer than dropping the fix, but the embedded field is not always nameable at the use site — it can be unexported and declared in another package — so this keeps the diagnostic and reports it without a fix instead.

The testdata gains both shapes. On the current tree the golden file fails with exactly the two invalid rewrites.