tests: Add a rewriter test that handles prefix

"prefix" has a lot of special handling that has to go on because so many
other options depend on prefix. It was also regressed by the option
refactor changes, so having a test feels appropriate.

This was verified against the 1.7 release, so it has the same behavior
as pre-option refactor Meson.

test cases/rewrite/7 prefix/meson.build

@@ -0,0 +1,5 @@
+project('lalala', 'cpp',
+    default_options : [
+        'prefix=/export/doocs',
+    ],
+)

unittests/rewritetests.py

@@ -407,3 +407,18 @@ class RewriterTests(BasePlatformTests):
         # Do it line per line because it is easier to debug like that
         for orig_line, new_line in zip_longest(original_contents.splitlines(), new_contents.splitlines()):
             self.assertEqual(orig_line, new_line)
+
+    @unittest.expectedFailure
+    def test_rewrite_prefix(self) -> None:
+        self.prime('7 prefix')
+        out = self.rewrite_raw(self.builddir, ['kwargs', 'info', 'project', '/'])
+        expected = {
+            'kwargs': {
+                'project#/': {
+                    "default_options": [
+                        'prefix=/export/doocs'
+                    ]
+                }
+            }
+        }
+        self.assertDictEqual(out, expected)