internal/refactor/inline: don't assume strings.Index calls internal/bytealg

I'm refactoring part of the strings package,
and this assumption won't hold after.

Change the test to use syscall.ByteSliceFromString;
syscall is mostly frozen and hopefully unlikely to change.

Change-Id: I0bb536b32922c6d705d0ff7369f03c55a169d37f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/581595
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: neild

internal/refactor/inline/testdata/std-internal.txtar

@@ -1,7 +1,7 @@
 
 std packages are a special case of the internal package check.
 
-This test assumes that strings.Index refers to internal/bytealg.
+This test assumes that syscall.ByteSliceFromString refers to internal/bytealg.
 
 -- go.mod --
 module testdata
@@ -10,6 +10,6 @@ go 1.12
 -- a/a.go --
 package a
 
-import "strings"
+import "syscall"
 
-var _ = strings.Index("", "") //@ inline(re"Index", re`inaccessible package "internal/bytealg"`)
+var _, _ = syscall.ByteSliceFromString("") //@ inline(re"ByteSliceFromString", re`inaccessible package "internal/bytealg"`)