[Tests] improve debug output on failure

Author: ljharb

test/install_script/nvm_check_global_modules

@@ -32,15 +32,17 @@ setup
 
 npm install -g nop >/dev/null || die 'nvm_check_global_modules cannot be tested because `npm` cannot install the `nop` package'
 message=$(nvm_check_global_modules)
-[ ! -z "$message" ] || die "nvm_check_global_modules should have printed a notice when npm had global modules installed"
+[ ! -z "$message" ] || die "nvm_check_global_modules should have printed a notice when npm had global modules installed; got:\n${message}"
 
-# Admit we're using NVM, just for this one test
-message=$(NVM_DIR=$ORIGINAL_NVM_DIR nvm_check_global_modules)
-[ -z "$message" ] || die "nvm_check_global_modules should not have printed a notice when npm is managed by nvm"
+if [ -n "${ORIGINAL_NVM_DIR}" ]; then
+  # Admit we're using NVM, just for this one test
+  message=$(NVM_DIR="${ORIGINAL_NVM_DIR}" nvm_check_global_modules)
+  [ -z "$message" ] || die "nvm_check_global_modules should not have printed a notice when npm is managed by nvm; got:\n${message}"
+fi
 
 npm uninstall -g nop >/dev/null
 message=$(nvm_check_global_modules)
-[ -z "$message" ] || die "nvm_check_global_modules should not have printed a notice when npm had no global modules installed"
+[ -z "$message" ] || die "nvm_check_global_modules should not have printed a notice when npm had no global modules installed; got:\n${message}"
 
 # Faking an installation of npm
 mkdir -p "$npm_config_prefix/lib/node_modules/npm"
@@ -49,15 +51,15 @@ cat <<'JSON' >"$npm_config_prefix/lib/node_modules/npm/package.json"
 JSON
 
 message=$(nvm_check_global_modules)
-[ -z "$message" ] || die "nvm_check_global_modules should have not printed a notice when npm had only itself installed as a global module"
+[ -z "$message" ] || die "nvm_check_global_modules should have not printed a notice when npm had only itself installed as a global module; got:\n${message}"
 
 # Faking the absence of npm
 PATH=".:$PATH"
 touch npm
 chmod +x npm
 
 message=$(nvm_check_global_modules)
-[ -z "$message" ] || die "nvm_check_global_modules should have not printed a notice when npm was unavailable"
+[ -z "$message" ] || die "nvm_check_global_modules should have not printed a notice when npm was unavailable; got:\n${message}"
 
 
 cleanup