Adding of a new exercise printalphabetalt.

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

go/tests/prog/printalphabetalt_prog/main.go

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+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("aBcDeFgHiJkLmNoPqRsTuVwXyZ")
+	fmt.Println("zYxWvUtSrQpOnMlKjIhGfEdCbA")
+}

go/tests/prog/printalphabetalt_test/main.go

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

subjects/printalphabetalt/README.md

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+## printalphabetalt
+
+### Instructions
+
+Write a program that:
+
+- first prints the Latin alphabet alternatively in lowercase and uppercase in order (from `'a'` to `'Z'`) on a single line.
+- second prints the Latin alphabet alternatively in lowercase and uppercase 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]]/printalphabetalt$ go build
+student@ubuntu:~/[[ROOT]]/printalphabetalt$ ./printalphabetalt
+aBcDeFgHiJkLmNoPqRsTuVwXyZ
+zYxWvUtSrQpOnMlKjIhGfEdCbA
+student@ubuntu:~/[[ROOT]]/printalphabetalt$
+```