internal/lsp: update unsafe completion test for upcoming spec changes

With unsafe.Slice about to be added, we'll get varying completions at
'unsafe.S_' depending on the Go version. Change the completion position
to be 'unsafe.Si_'.

Change-Id: Ib537fefceda7864b7a256565a3a7286d18e845c6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/312470
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

internal/lsp/testdata/unsafe/unsafe.go

@@ -9,6 +9,5 @@ import (
 
 func _() {
 	x := struct{}{}
-	_ = unsafe.Sizeof(x) //@complete("i", Sizeof)
+	_ = unsafe.Sizeof(x) //@complete("z", Sizeof)
 }
-