Make error message when embedding version test fails more friendly

Author: ngoldbaum

testing/cffi0/test_version.py

@@ -59,4 +59,8 @@ def test_embedding_h():
     v = cffi.__version__
     p = cffi_root / 'src/cffi/_embedding.h'
     content = _read(p)
-    assert ('cffi version: %s"' % (v,)) in content
+    loc = content.find('cffi version: ')
+    assert loc > 0, "Cannot find cffi version string in _embedding.h"
+    context = content[loc-100:loc+100]
+    msg = f"CFFI verison is incorrect, context for current version string is:\n{context}"
+    assert (f'cffi version: {v}"') in context, msg