Adding of a new exercise printalphabetalt2.

The goal: printing the alphabet alternatively in uppercase and in lowercase from A to z, and then from Z to a.

go/tests/prog/printalphabetalt2_prog/main.go

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+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("AbCdEfGhIjKlMnOpQrStUvWxYz")
+	fmt.Println("ZyXwVuTsRqPoNmLkJiHgFeDcBa")
+}

go/tests/prog/printalphabetalt2_test/main.go

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+package main
+
+import (
+	"lib"
+)
+
+func main() {
+	lib.ChallengeMain("printalphabetalt2")
+}

subjects/printalphabetalt2/README.md

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+## printalphabetalt2
+
+### Instructions
+
+Write a program that:
+
+- first prints the Latin alphabet alternatively in uppercase and lowercase in order (from `'A'` to `'z'`) on a single line.
+- second prints the Latin alphabet alternatively in uppercase and lowercase in reverse order (from `'Z'` to `'a'`) on a single line.
+
+A line is a sequence of characters preceding the [end of line](https://en.wikipedia.org/wiki/Newline) character (`'\n'`).
+
+### Usage
+
+```console
+student@ubuntu:~/[[ROOT]]/printalphabetalt2$ go build
+student@ubuntu:~/[[ROOT]]/printalphabetalt2$ ./printalphabetalt2
+AbCdEfGhIjKlMnOpQrStUvWxYz
+ZyXwVuTsRqPoNmLkJiHgFeDcBa
+student@ubuntu:~/[[ROOT]]/printalphabetalt2$
+```