Reduce execution time of some C tests by reducing the number of
iterations to a more reasonable level.

* subversion/tests/libsvn_fs_x/string-table-test.c
  (many_strings_table_body): Having over 50 sub-tables is no better
                             than 6 sub-tables when we simply want
                             to test for large strings and multiple
                             sub-tables.
* subversion/tests/libsvn_subr/named_atomic-test.c
  (calibrate_iterations): Limiting the number of iterations to 250k
                          still gives plenty of opportunity for faulty
                          sync to show up.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545937 13f79535-47bb-0310-9956-ffa450edef68