🦺 Transfer remaining Manifold tests to the new test suite

[kellertuer]

This continues #839 and the remainder of the list in #847 with the goal of solely using the new test suite and finding the last few code cov lines to cover them as well (at time of writing 6 lines left).
Instead of the global PR for all, this is meant as a list to keep track of.

This is also a very good starting point if you notice a manifold you like to work with and want to get to know its features, maybe consider transferring ists tests to the new suite.

Manifold tests to move to the new test suite

• direct_sum_bundle
• elliptope
• embedded_torus
• essential_manifold
• euclidean
• fiber_bundle
• fiber
• fixed_rank
• flag
• generalized_grassmann
• generalized_stiefel
• graph
• grassmann
• hamiltonian
• hyperbolic
• hyperrectangle
• invertible_matrices
• lorentz
• multinomial_doubly_stochastic
• multinomial_matrices
• multinomial_spd
• multinomial_symmetric
• oblique
• positive_numbers
• power_manifold
• probability_simplex
• product_manifold
• projective_space
• rotations
• segre
• shape_space
• skewhermitian
• spd_fixed_determinant
• spectrahedron
• sphere_symmetric_matrices
• symmetric_positive_definite
• symmetric_positive_semidefinite_fixed_rank
• symmetric
• symplectic
• symplecticgrassmann
• symplecticstiefel
• torus
• tucker
• vector_bundle

Tests to check / transfer

These are currently used within the old tests, so as soon as these are no longer called they should be removed

• find_eps
• test_parallel_transport
• test_manifold
• function test_atlases

Maybe also implement the tests for

These are tests we can maybe implement along the way of further functions in the API -

• test_estimated_sectional_curvature_matrix
• test_sectional_curvature_matrix
• test_affine_connection
• test_get_atlas
• test_get_parameters
• test_get_point
• test_transition_map
• test_transition_map_diff
• test_induced_basis
• test_check_chart_switch
• test_get_chart_index
• add second variant inner(M::AbstractManifold, A::AbstractAtlas, i, a, Xc, Yc)
• add second variant norm(M::AbstractManifold, A::AbstractAtlas, i, a, Xc)
• test_mean
• test_median

Usability

• add more examples, especially to the test_manifold doc