Add `nvm_ensure_version_prefix` unit tests.

test/fast/Unit tests/nvm_ensure_version_prefix

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+die () { echo $@ ; exit 1; }
+
+. ../../../nvm.sh
+
+[ "_$(nvm_ensure_version_prefix 1)" = "_v1" ] || die '"nvm_ensure_version_prefix 1" did not return "v1"'
+[ "_$(nvm_ensure_version_prefix v1)" = "_v1" ] || die '"nvm_ensure_version_prefix v1" did not return "v1"'
+[ "_$(nvm_ensure_version_prefix foo)" = "_foo" ] || die '"nvm_ensure_version_prefix foo" did not return "foo"'