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