Test suite: reference plots come from files, not inline

This makes maintenance infinitely easier

t/3d-spiral-with-bounds-labels-square-xy-aspect-ratio.ref

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+                                           *AA*
+                                               AA*A
+                                                   A
+                                        +           A*A
+                                    40  |+             A
+                                        |               A
+                                    30  |+               A
+                                        | AAAAA*          A
+                                 z  20  |+AA    AA        A
+                                        |  AA*    A*A    AA
+                                        |     AA*AAA*AA*A
+                                    10  |+             AAA
+                                     0  |+                A
+                                        |                 A
+                                        |    +-           A
+                                        |   -++---
+                                        | --+    +---
+                                        |-++        +---
+                                        +++--           ---
+                                       -21.5+---          +- 1
+                                           -10.5---      + 0.6
+                                                0 +---  +  024
+                                                 0.5  ++-0.4
+                                                    1.521.8
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