Remove `test_solver`

[mkovari]

The routine test_solver in the integration tests takes a long time to run. Moreover, I don't know that it is very useful. Tim's comments in the code:
# TODO: I'm not sure how useful this test is anymore? TN
and at the end,
# TODO This might need to assert something, like ifail == 1?

Doc string:

Code to test PROCESS Solver by choosing different starting values
    for the iteration parameters around the initial values in the INPUT file
    Code modified by Sarah Medley in April 2015 to also calculate Q (fusion power/
    injected power) and output this to SolverTest.out

For the moment I will change the NITER, the number of times PROCESS is run, from 10 to 1 in integration/data/test_solver.conf.
Would anyone complain if I remove the test completely?
@timothy-nunn @jonmaddock @jmorris-uk