t9010: svnadmin can fail even if available

t9010: svnadmin can fail even if available

If svn is built against one version of SQLite and run against another,
libsvn_subr needlessly errors out in operations that need to make a
commit.

That is clearly not a bug in git but let us consider the ramifications for
the test suite.  git-svn uses libsvn directly and is probably broken by
that bug; it is right for git-svn tests to fail.  The vcs-svn lib, on the
other hand, does not use libsvn and the test t9010 only uses svn to check
its work.  This points to two possible improvements:

 - do not disable most vcs-svn tests if svn is missing.
 - skip validation rather than failing it when svn fails.

Bring about both by putting the svn invocations into a single test that
builds a repo to compare the test-svn-fe result against.  The test will
always pass but only will set the new SVNREPO test prereq if svn succeeds;
and validation using that repo gets an SVNREPO prerequisite so it only
runs with working svn installations.

Works-around: http://bugs.debian.org/608925
Noticed-by: A Large Angry SCM <gitzilla@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>