[New] add lts/-1 etc, to point to an LTS line relative to "latest"

test/fast/Unit tests/mocks/LTS_names.txt

@@ -0,0 +1,5 @@
+erbium
+dubnium
+carbon
+boron
+argon

test/fast/Unit tests/nvm_alias LTS-N

@@ -0,0 +1,34 @@
+#!/bin/sh
+
+die () { echo "$@" ; cleanup ; exit 1; }
+
+
+cleanup() {
+  unset -f nvm_download
+}
+
+\. ../../../nvm.sh
+
+set -ex
+
+MOCKS_DIR="${PWD}/mocks"
+
+# sample output at the time the test was written
+TAB_PATH="${MOCKS_DIR}/nodejs.org-dist-index.tab"
+nvm_download() {
+  cat "${TAB_PATH}"
+}
+
+nvm_ls_remote >/dev/null || die "nvm_ls_remote_failed?!"
+
+LTS_NAMES_PATH="${MOCKS_DIR}/LTS_names.txt"
+
+N=0
+while IFS= read -r LTS; do
+  N=$(($N+1))
+  EXPECTED="$(nvm_alias "lts/${LTS}")"
+  ACTUAL="$(nvm_alias "lts/-${N}")"
+  [ "${EXPECTED}" = "${ACTUAL}" ] || die "\`nvm_alias lts/-${N}\` was \`${ACTUAL}\`; expected \`${EXPECTED}\`"
+done < "${LTS_NAMES_PATH}"
+
+cleanup