Add a unit test for svn_error_purge_tracing() that asserts that SVN_ERR_ASSERT() fails when an error chain with only tracing errors is passed in, which shouldn't happen in practice.
Add a unit test for svn_error_purge_tracing() that asserts that
SVN_ERR_ASSERT() fails when an error chain with only tracing errors is
passed in, which shouldn't happen in practice.

* subversion/tests/libsvn_subr/error-test.c
  (test_error_purge_tracing):
    Only if SVN_ERR__TRACING is defined, build an error chain with
    only tracing errors and pass it to svn_error_purge_tracing(),
    asserting that it returns a new error chain with a
    SVN_ERR_ASSERTION_FAIL error.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion