[tests] relocated test files from examples/testdata to internal/boxcli (#540)

## Summary
- Moved test files for devbox commands to follow the convention of
`sample.go` and `sample_test.go` being in the same directory.
- Made small improvements to the framework:
  - Added `SetEnv()` 
  - Added `CreateFile()`
  - Updated `SetDevboxjson`
  - Added `Close()`
- Updated framework so that it runs the tests in a `test_tmp_*`
directory and cleans it up after tests are done.
 
## How was it tested?
Run tests in any test file in `intenal/boxcli`.
Note: shell_test.go already was there so I appended my tests to the file
instead of creating a new file.

Author: mohsenari

.gitignore

@@ -32,4 +32,4 @@ __pycache__/
 *.devcontainer
 
 # test specific files
-examples/testdata/commands/generate/Dockerfile
+.test_tmp_*