Extract `mojo_test` - standalone apptest runner in devtools.

mojo/devtools/common/README.md

 # Devtools
 
 Unopinionated tools for **running**, **debugging** and **testing** Mojo apps.
 
 ## Install
 
 ```
 git clone https://github.com/domokit/devtools.git
 ```
 
 ## Contents
 
 Devtools offers the following tools:
 
- - `mojo_run` - universal shell runner
- - `mojo_debug` - supports interactive tracing and debugging of a running mojo
-   shell
- - `remote_adb_setup` - configures adb on a remote machine to communicate with
-   a device attached to the local machine
+ - `mojo_run` - shell runner
+ - `mojo_test` - apptest runner
+ - `mojo_debug` - debugger supporting interactive tracing and debugging of a
+   running mojo shell
 
-and a Python scripting library designed for being embedded (`devtoolslib`).
+Additionally, `remote_adb_setup` script helps to configure adb on a remote
+machine to communicate with a device attached to a local machine, forwarding the
+ports used by `mojo_run`.
 
 ### Debugger
 
 The `mojo_debug` script allows you to interactively inspect a running shell,
 collect performance traces and attach a gdb debugger.
 
 #### Tracing
 To collect [performance
 traces](https://www.chromium.org/developers/how-tos/trace-event-profiling-tool)
 and retrieve the result:
 
 ```sh
 mojo_debug tracing start
 mojo_debug tracing stop [result.json]
 ```
 
 The trace file can be then loaded using the trace viewer in Chrome available at
 `about://tracing`.
 
 #### GDB
-It is possible to inspect a Mojo Shell process using GDB. The `mojo_debug` script
-can be used to launch GDB and attach it to a running shell process (android
-only):
+It is possible to inspect a Mojo Shell process using GDB. The `mojo_debug`
+script can be used to launch GDB and attach it to a running shell process
+(android only):
 
 ```sh
 mojo_debug gdb attach
 ```
 
 Once started, GDB will first stop the Mojo Shell execution, then load symbols
 from loaded Mojo applications. Please note that this initial step can take some
 time (up to several minutes in the worst case).
 
 After each execution pause, GDB will update the set of loaded symbols based on
 the selected thread only. If you need symbols for all threads, use the
 `update-symbols` GDB command:
 ```sh
 (gdb) update-symbols
 ```
 
 If you only want to update symbols for the current selected thread (for example,
 after changing threads), use the `current` option:
 ```sh
 (gdb) update-symbols current
 ```
 
 #### Android crash stacks
 When Mojo shell crashes on Android ("Unfortunately, Mojo shell has stopped.")
-due to a crash in native code, `mojo_debug` can be used to find and symbolize the
-stack trace present in the device log:
+due to a crash in native code, `mojo_debug` can be used to find and symbolize
+the stack trace present in the device log:
 
 ```sh
 mojo_debug device stack
 ```
 
 ### devtoolslib
 
 **devtoolslib** is a Python module containing the core scripting functionality
 for running Mojo apps: shell abstraction with implementations for Android and
 Linux and support for apptest frameworks. The executable scripts in devtools are
 based on this module. One can also choose to embed the functionality provided by
 **devtoolslib** in their own wrapper.
 
 ## Development
 
 The library is canonically developed [in the mojo
 repository](https://github.com/domokit/mojo/tree/master/mojo/devtools/common),
 https://github.com/domokit/devtools is a mirror allowing to consume it
 separately.