[Tests] colors: pin NVM_HAS_COLORS to stay local, and pin the invalid-.nvmrc message

Address ljharb's review: assert `nvm --help` and `nvm set-colors` don't
leak the `local NVM_HAS_COLORS` guard into the calling shell, and assert
`nvm_nvmrc_invalid_msg` still emits its actual message text, not just
that it's uncolored.

Author: lazerg

test/fast/Set Colors/nvm --help colors the set-colors legend

@@ -51,4 +51,7 @@ case "${HELP}" in
   *"${EXPECTED}"*) die "nvm --help --no-colors colored the legend" ;;
 esac
 
+nvm --help >/dev/null
+[ -z "${NVM_HAS_COLORS-}" ] || die "nvm --help leaked NVM_HAS_COLORS=${NVM_HAS_COLORS} into the shell"
+
 cleanup

test/fast/Set Colors/nvm set-colors colors the confirmation line

@@ -29,4 +29,9 @@ EXPECTED_OUTPUT="$(command printf %b 'Setting colors to: \033[0;34mb\033[0m\033[
 
 [ "_${OUTPUT}" = "_${EXPECTED_OUTPUT}" ] || die "expected >${EXPECTED_OUTPUT}<; got >${OUTPUT}<"
 
+rm -f "${NVM_HAS_COLORS_MARKER}"
+NVM_HAS_COLORS_MARKER="$(mktemp)"
+nvm set-colors bygre >/dev/null
+[ -z "${NVM_HAS_COLORS-}" ] || die "nvm set-colors leaked NVM_HAS_COLORS=${NVM_HAS_COLORS} into the shell"
+
 cleanup

test/fast/Unit tests/nvm_nvmrc_invalid_msg

@@ -17,4 +17,9 @@ case "${OUTPUT}" in
   *"${ESC}"*) die "an exported NVM_HAS_COLORS should not color the invalid .nvmrc message; got >${OUTPUT}<" ;;
 esac
 
+case "${OUTPUT}" in
+  *'invalid .nvmrc!'*) ;;
+  *) die "expected the invalid-.nvmrc message; got >${OUTPUT}<" ;;
+esac
+
 cleanup