Speed up top 10 slowest tests by using coarser grids or fewer timesteps

[anushkasinghh]

This PR reduces the runtime of the 10 slowest tests by adjusting test configurations:

• coarser grids
• lower polynomial degree
• fewer timesteps

In addition, these tests are grouped under:psydac/api/tests/test_ci_performance.py so they can be called separately if and when needed.
The goal is to keep tests meaningful while cutting down execution time.

Test	New branch (s)	Old commit (s)	Δ (new–old)
test_maxwell_2d_2_patch_dirichlet_1	288.25	261.26	+26.99
test_navier_stokes_2d	111.99	119.58	–7.59
test_poisson_pretzel_f	96.62	95.89	+0.73
test_poisson_pretzel_f_nc	95.52	100.23	–4.71
test_time_harmonic_maxwell_pretzel_f	91.82	96.90	–5.08
test_time_harmonic_maxwell_pretzel_f_nc	93.00	104.25	–11.25
test_maxwell_3d_1	83.85	73.63	+10.22
test_maxwell_3d_2_mult	50.95	89.03	–38.08
test_maxwell_2d_2_patch_dirichlet_0	46.26	44.71	+1.55
test_maxwell_2d_2_patch_dirichlet_2	23.47	25.29	–1.82

To summarize:

• New branch: 983.21s (~16m 23s)
• Earlier: 1012.67s (~16m 52s)
• Net gain: ~29.5s faster

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00%	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (42b214e)	30762	18640	60.59%
Head commit (d5b085a)	61524 (+30762)	37278 (+18638)	60.59% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#521)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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