Modularize testing

[iamashwin99]

• Move the sourcing of spack, creating env, running the spec, installing the spec and testing it into a separate bash script
• allows using the spack_test_install.sh multiple times for different specs ( for eg cmake, different compilers etc ) with less code duplication in the future
• allows using the spack_test_install.sh outside the repo for manual testing
• allows creation of a CI that tests the installation of the spack upstream package definition as well ( checks for eg when the download links fail or for changes in spack)

[iamashwin99]

The failed build in spack-develop-octopus-develop is due to a problem in octopus main branch and is independent of this MR. see for eg a CI run on current the develop https://github.com/fangohr/octopus-in-spack/actions/runs/9406513594/job/25910128602 which also fails.

Reported at https://gitlab.com/octopus-code/octopus/-/issues/1029

[iamashwin99]

@fangohr, given that the test failures are due to some changes in octopus develop, which have already been reported upstream, what should our strategy be for this and following MRs?