Keep SPECTRE phiedge in the axis_volume test helper (#612)

SIMPLE main is red since [libneo](https://github.com/itpplasma/libneo)
#444 (99f86323), which makes the SPECTRE reader require
`input/physics/phiedge`. `axis_volume()` in
`test/tests/spectre_test_utils.py` rewrites the fixture to a single
volume but dropped that scalar, so `simple.x` stopped with `HDF5 object
not found: phiedge` in test_spectre_gc_rk45, test_spectre_sympl_volume,
test_spectre_construction_grid and test_spectre_validation (and the
_full variants). The helper now copies the global edge flux unchanged,
as real SPECTRE output always carries it; oracles are untouched.

Chris&AI

Author: krystophny

test/tests/spectre_test_utils.py

@@ -32,7 +32,11 @@ def write(key, values):
 
             for key in ("input/physics/Igeometry", "input/physics/Nfp",
                         "input/physics/Mpol",
-                        "input/physics/Ntor", "output/mn", "output/im", "output/in"):
+                        "input/physics/Ntor", "input/physics/phiedge",
+                        "output/mn", "output/im", "output/in"):
+                # phiedge is SPECTRE's global edge toroidal flux, kept unchanged
+                # because real SPECTRE output always carries it and libneo's
+                # reader requires it.
                 write(key, src[key][:])
             for key in ("input/physics/Nvol", "output/Mvol"):
                 write(key, [1])