BUG: Tests in test_eigensolver do not check for SLEPc

[JDBetteridge]

Describe the bug
Tests in tests/regression/test_eigensolver.py should check for the presence of SLEPc.

Steps to Reproduce
Steps to reproduce the behaviour:

1. Install Firedrake without SLEPc
2. pytest -v tests/regression/test_eigensolver.py

Expected behavior
Tests skipped

Error message

FAILED tests/regression/test_eigensolver.py::test_evals_1d[5-1-1e-13] - ImportError: Unable to import SLEPc, eigenvalue computation not possible (try firedrake-update --slepc)
FAILED tests/regression/test_eigensolver.py::test_evals_1d[10-1-1e-13] - ImportError: Unable to import SLEPc, eigenvalue computation not possible (try firedrake-update --slepc)
FAILED tests/regression/test_eigensolver.py::test_evals_1d[20-1-1e-13] - ImportError: Unable to import SLEPc, eigenvalue computation not possible (try firedrake-update --slepc)
FAILED tests/regression/test_eigensolver.py::test_evals_1d[30-1-1e-13] - ImportError: Unable to import SLEPc, eigenvalue computation not possible (try firedrake-update --slepc)
FAILED tests/regression/test_eigensolver.py::test_evals_2d - ImportError: Unable to import SLEPc, eigenvalue computation not possible (try firedrake-update --slepc)

Environment:

• N/A

Additional Info
N/A