[Tests] `nvm_command_info`: make error output more helpful

test/fast/Unit tests/nvm_command_info

@@ -46,9 +46,13 @@ WGET_EXPECTED_INFO="$(type wget)"
 cleanup
 
 # 4. nvm_command_info() should not have standard error
-nvm_command_info ls   2>&1 >/dev/null | grep . && die "\`nvm_command_info ls\`   should not return standard error message(stage 4)"
-nvm_command_info rm   2>&1 >/dev/null | grep . && die "\`nvm_command_info rm\`   should not return standard error message(stage 4)"
-nvm_command_info git  2>&1 >/dev/null | grep . && die "\`nvm_command_info git\`  should not return standard error message(stage 4)"
-nvm_command_info grep 2>&1 >/dev/null | grep . && die "\`nvm_command_info grep\` should not return standard error message(stage 4)"
+OUTPUT="$(nvm_command_info ls   2>&1 >/dev/null)"
+[ -z "${OUTPUT}" ] || die "\`nvm_command_info ls\`   expected no stderr; got >${OUTPUT}< (stage 4)"
+OUTPUT="$(nvm_command_info rm   2>&1 >/dev/null)"
+[ -z "${OUTPUT}" ] || die "\`nvm_command_info rm\`   expected no stderr; got >${OUTPUT}< (stage 4)"
+OUTPUT="$(nvm_command_info git  2>&1 >/dev/null)"
+[ -z "${OUTPUT}" ] || die "\`nvm_command_info git\`  expected no stderr; got >${OUTPUT}< (stage 4)"
+OUTPUT="$(nvm_command_info grep 2>&1 >/dev/null)"
+[ -z "${OUTPUT}" ] || die "\`nvm_command_info grep\` expected no stderr; got >${OUTPUT}< (stage 4)"
 
 cleanup