tests: work around libmagic bug in svn subrepo tests

libmagic 5.40 introduced a bug [1] wherein ASCII text files with fewer
than 3 distinct character values would be reported as binary data
rather than as text. This bug was later fixed in version 5.41 [2].

SVN uses libmagic to determine the MIME type of added files with missing
or unknown extensions [3]. This results in test failures on systems with
libmagic 5.40 installed:

   $ echo a > a
   $ svn add a
-  A         a
+  A  (bin)  a

A simple workaround is to change the test file's content to include
3 distinct ASCII values (including the terminating newline).

[1] https://bugs.astron.com/view.php?id=180
[2] https://bugs.astron.com/view.php?id=261
[3] https://svnbook.red-bean.com/en/1.8/svn.advanced.props.html#idm2649
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