Adding of a new exercise printreversealphabetg.

The goal: printing the alphabet in lowercase from g to a.

go/tests/prog/printreversealphabetg_prog/main.go

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

go/tests/prog/printreversealphabetg_test/main.go

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

subjects/printreversealphabetg/README.md

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+## printreversealphabetg
+
+### Instructions
+
+Write a program that prints the Latin alphabet in lowercase in reverse order (from `'g'` 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'`).
+
+Please note that `casting` is not allowed for this exercise!
+
+### Usage
+
+```console
+student@ubuntu:~/[[ROOT]]/printreversealphabetg$ go build
+student@ubuntu:~/[[ROOT]]/printreversealphabetg$ ./printreversealphabetg
+gfedcba
+student@ubuntu:~/[[ROOT]]/printreversealphabetg$
+```