[Refactor] add nvm_get_artifact_compression

test/fast/Unit tests/nvm_get_artifact_compression

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+cleanup () {
+  unset -f die cleanup
+}
+die () { echo "$@" ; cleanup ; exit 1; }
+
+\. ../../../nvm.sh
+
+# nvm_get_artifact_compression by default
+[ "$(nvm_get_artifact_compression)" = 'tar.gz' ] || die 'nvm_get_artifact_compression should return "tar.gz" by default'
+
+# nvm_get_artifact_compression with xz
+[ "$(nvm_get_artifact_compression "14.0.0")" = 'tar.xz' ] || die 'nvm_get_artifact_compression should return "tar.xz" for this version'
+
+cleanup