tests: escape backslash in makepatch.py inline file

This avoids some SyntaxWarning on Python 3.8 due to unescaped \.

Differential Revision: https://phab.mercurial-scm.org/D5840

tests/test-import-eol.t

@@ -17,9 +17,9 b''
   >    'empty:stripped-crlf': b'\r\n'}[sys.argv[1]])
   > w(b' d\n')
   > w(b'-e\n')
-  > w(b'\ No newline at end of file\n')
+  > w(b'\\ No newline at end of file\n')
   > w(b'+z\r\n')
-  > w(b'\ No newline at end of file\r\n')
+  > w(b'\\ No newline at end of file\r\n')
   > EOF
 
   $ hg init repo

tests/test-mq-eol.t

@@ -23,9 +23,9 b' Test interactions between mq and patch.e'
   > w(b' c\r\n')
   > w(b' d\n')
   > w(b'-e\n')
-  > w(b'\ No newline at end of file\n')
+  > w(b'\\ No newline at end of file\n')
   > w(b'+z\r\n')
-  > w(b'\ No newline at end of file\r\n')
+  > w(b'\\ No newline at end of file\r\n')
   > EOF
 
   $ cat > cateol.py <<EOF