* spack test: subcommands for asynchronous tests

* commands are `run`, `list`, `status`, `results`, `remove`.

* tests: Preliminary libsigsegv smoke tests (updated)

* Cleaned up and added doc to libsigsegv smoke test

* tests: Update openmpi smoke tests to new run_test api

* Removed version check try-except tracking per discussion

* Changed openmpi orted command status values to list

* install tests: added support for multiple test command status values

* Initial libxml2 tests (using executables)

* Expanded libxml2 tests using installed bins

* Refactored/generalized _run_tests

Existing test methods run via callbacks at install time when run with `spack install --run-tests`
These methods are tied into the package build system, and cannot be run arbitrarily
New naming scheme for these tests based on the build system phase after which they should be run
The method name `test` is now reserved for methods run via the `spack test` command

* Added install test for perl, including use statements