testdata: make pointer regexp less prone to flakes

Pointers are of the form "0x" and a number of hex characters.
Those are all part of the obfuscated filename alphabet,
so if we're unlucky enough, we can get hits:

	> cmp stdout reverse.stdout
	--- stdout
	+++ reverse.stdout
	@@ -4,7 +4,7 @@
	 runtime/debug.Stack(...)
		runtime/debug/stack.go:24 +0x??
	 test/main/lib.printStackTrace(...)
	-	pv0x??mRa.go:1 +0x??
	+	test/main/lib/long_lib.go:32 +0x??
	 test/main/lib.(*ExportedLibType).ExportedLibMethod(...)
		test/main/lib/long_lib.go:19 +0x??
	 main.unexportedMainFunc.func1(...)

Include the plus sign in the regular expression,
which is used for showing pointers but isn't part of our alphabet.

testdata/scripts/reverse.txt

@@ -110,13 +110,13 @@ func printStackTrace(w io.Writer) error {
 	// The format also changes depending on the ABI.
 	// Strip them out here, to have portable static stdout files.
 	rxCallArgs := regexp.MustCompile(`\(({|0x)[^)]+\)|\(\)`)
-	rxPointer := regexp.MustCompile(`0x[0-9a-f]+`)
+	rxPointer := regexp.MustCompile(`\+0x[0-9a-f]+`)
 
 	// Keep this comment here, because comments affect line numbers.
 
 	stack := debug.Stack()
 	stack = rxCallArgs.ReplaceAll(stack, []byte("(...)"))
-	stack = rxPointer.ReplaceAll(stack, []byte("0x??"))
+	stack = rxPointer.ReplaceAll(stack, []byte("+0x??"))
 	_, err := w.Write(stack)
 	return err
 }