[TEST] Repair seqan2 tests

[Irallia]

This PR is part of seqan/product_backlog#12.
It resolves the errors with seqan2, such as

.../seqan3/test/performance/range/views/view_translate_2D_1D_benchmark.cpp: In function 'void copy(benchmark::State&)':
.../seqan3/test/performance/range/views/view_translate_2D_1D_benchmark.cpp:195:38: error: 'generate_sequence_seqan' was not declared in this scope; did you mean 'seqan3::test::generate_sequence_seqan2'?
  195 |         dna_sequence_collection[i] = generate_sequence_seqan2<seqan::Dna>(100, 0, 0);
      |                                      ^~~~~~~~~~~~~~~~~~~~~~~~
      |                                      seqan3::test::generate_sequence_seqan2

and

.../seqan3/test/performance/alignment/global_affine_alignment_parallel_benchmark.cpp: In function 'void seqan2_affine_dna4_parallel(benchmark::State&)':
.../seqan3/test/performance/alignment/global_affine_alignment_parallel_benchmark.cpp:144:5: error: 'ExecutionPolic' was not declared in this scope; did you mean 'seqan::ExecutionPolicy'?
  144 |     ExecutionPolicy<Parallel, Serial> exec{};
      |     ^~~~~~~~~~~~~~~
      |     seqan::ExecutionPolicy

when the seqan submodule is available.

[codecov]

Codecov Report

Merging #1995 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1995   +/-   ##
=======================================
  Coverage   97.89%   97.89%           
=======================================
  Files         262      262           
  Lines        9865     9865           
=======================================
  Hits         9657     9657           
  Misses        208      208           

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