Added test for hp-constraints on Q elements with Gauss-Lobatto support points.

[marcfehling]

Since #2462 we test the interpolation behavior of Q elements only on equidistant support points.

I suggest that we should bring this check back as a separate test.

I explicitly set the support points to come from the QGaussLobatto formula, so that this test is independent of the default choice for support points of Q elements.

---

@bangerth, @kronbichler, @peterrum We were interested in checking the following configuration in 3D

+-----+---+---+
|     |p=2|p=2|
| p=3 +---+---+
|     |p=2|p=2|
+-----+---+---+

for which the interpolation checks yield the following results.

DEAL::Testing FE_Q<3>(3) vs. FE_Q<3>(2)
DEAL::  Relative interpolation error before constraints: 1.44112e-17
DEAL::  Relative difference after constraints: 1.77206e-17
DEAL::  Relative interpolation error before constraints: 1.94552e-17
DEAL::  Relative difference after constraints: 4.45218e-17
DEAL::  Relative interpolation error before constraints: 1.93839e-17
DEAL::  Relative difference after constraints: 4.64376e-17
DEAL::  Relative interpolation error before constraints: 2.04974e-17
DEAL::  Relative difference after constraints: 2.14569e-17

I believe that confirms that our constraints are working.

[bangerth]

This patch adds 79k lines (~2MB) of output. But then there are several related tests already that output the same amount of information, so I'll let it go this time. Most of these output files are highly repetitive and should compress pretty well in the git cache. In any case, if we change the repetitive part, we should do that in the test_*() functions, but that has to happen in the .h file that is common to all of these tests.

[kronbichler]

Indeed, these tests are huge, with most of the output coming from the 3d case. While it is not wrong to thoroughly test the 3d case as well, I think that we ought look into those at some point. It seems we were concerned with a similar question in #2462 almost 8 years ago, so we should audit the test indeed.

[kronbichler]

But this is a rather small addition, so let us not both with the present PR.