* dev-build: --drop-in <shell>
Add a `--drop-in <shell>` option to `spack dev-build`.
This option will automatically run a
`spack build-env <spec> -- <shell>` at the end of a `dev-build`, e.g.
to quickly drop-and-devel into a build phase of a package.
Example usage:
```
spack dev-build --before cmake --drop-in bash openpmd-api@develop
```
* build_env: drop in unit test
Co-authored-by: Greg Becker <becker33@llnl.gov>