[tests.linearelliptic] add more test cases

[ftalbrecht]

I found another one in the diploma thesis of S. Kaulmann (Section 6.1.1): taking \lambda = \kappa = 1 and f = -q (q - 1) x^{q - 2} on the unit cube in 2d with Dirichlet boundary values g_D = x^q everywhere (for any q \in \N) yields the exact solution p(x) = x^q.

[ftalbrecht]

Another two are in ER2007, page 858 (we already use the 2d one):

• 1d: exact solution cos(8 pi x)
• 3d: exact solution cos(8 pi x) cos(8 pi y) cos(8 pi z)

[renefritze]

You've add the ER2007 case since posting. I do think the polynomial one would still be nice though.

[ftalbrecht]

Agreed, will do. Perhaps after #97 is done, recording the results should be nicer then.

[tobiasleibner]

Moved to gitlab.