adding placeholders for matrix_error_handling tests
moving check_not_nan tests out of math/matrix_error_handling_test.cpp
moving check_unit_vector tests out of math/matrix_error_handling_test.cpp
moving check_not_nan tests out of math/matrix_error_handling_test.cpp
moving check_simplex tests out of math/matrix_error_handling_test.cpp
moving check_symmetric tests out of math/matrix_error_handling_test.cpp
moving check_cov_matrix tests out of math/matrix_error_handling_test.cpp
moving check_cov_matrix tests out of math/matrix_error_handling_test.cpp
moving check_consistent_size, check_consistent_sizes tests out of math/matrix_error_handling_test.cpp
removing unused test
removing unused src/stan/math/error_handling/matrix/check_not_nan.hpp