Return error when no command is passed

[nprizal]

Description

bktec requires a command (run or plan) from version 2.0.0. Currently, running bktec without a command prints help text and exits with 0.

$ bktec
NAME:
   bktec - Buildkite Test Engine Client

USAGE:
   bktec [global options] [command [command options]]

COMMANDS:
   run      Run tests
   plan     Generate test plan without running tests
   help, h  Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --version   print version information and exit
   --debug     Enable debug output [$BUILDKITE_TEST_ENGINE_DEBUG_ENABLED]
   --help, -h  show help
$ echo $?
0

Users upgrading from 1.x.x to 2.x.x without specifying the command will see their build pass even though no tests are being run. Therefore, we should return a non-zero exit code when no command is provided to let users know they need to set the command.

Testing

I've tested locally without subcommand and an invalid command.

$ bktec
NAME:
   bktec - Buildkite Test Engine Client

USAGE:
   bktec [global options] [command [command options]]

COMMANDS:
   run      Run tests
   plan     Generate test plan without running tests
   help, h  Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --version   print version information and exit
   --debug     Enable debug output [$BUILDKITE_TEST_ENGINE_DEBUG_ENABLED]
   --help, -h  show help

command is required
$ echo $?
16

$ bktec foo
NAME:
   bktec - Buildkite Test Engine Client

USAGE:
   bktec [global options] [command [command options]]

COMMANDS:
   run      Run tests
   plan     Generate test plan without running tests
   help, h  Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --version   print version information and exit
   --debug     Enable debug output [$BUILDKITE_TEST_ENGINE_DEBUG_ENABLED]
   --help, -h  show help

invalid command: "foo"
$ echo $?
16