Non constexpr expressions in tests

[rbberger]

Tests fail to compile due to non-constexpr parts:

[ 85%] Building CXX object test/CMakeFiles/test_portsofcall.dir/test_static_vector.cpp.o
oss/ports-of-call/test/test_span.cpp:567:24: error: constexpr variable 'pi' must be initialized by a constant expression
  567 |   constexpr const Real pi = std::acos(-1);
      |                        ^    ~~~~~~~~~~~~~
/opt/rh/gcc-toolset-12/root/usr/lib/gcc/x86_64-redhat-linux/12/../../../../include/c++/12/cmath:100:14: note: subexpression not valid in a constant expression
  100 |     { return __builtin_acos(__x); }
      |              ^
oss/ports-of-call/test/test_span.cpp:567:29: note: in call to 'acos(-1)'
  567 |   constexpr const Real pi = std::acos(-1);
oss/ports-of-call/test/test_span.cpp:569:18: error: constexpr variable 'd_gp' must be initialized by a constant expression
  569 |   constexpr Real d_gp = 2. * pi / static_cast<Real>(N);
      |                  ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
oss/ports-of-call/test/test_span.cpp:569:30: note: initializer of 'pi' is not a constant expression
  569 |   constexpr Real d_gp = 2. * pi / static_cast<Real>(N);
      |                              ^
oss/ports-of-call/test/test_span.cpp:567:24: note: declared here
  567 |   constexpr const Real pi = std::acos(-1);
      |                        ^
2 errors generated when compiling for gfx942.