Test framework for C tests - svn_test__sandbox_create()

[Julian Foad]

I keep coming across simple bugs in untested APIs, and sometimes I try
to write simple unit tests for those APIs.  To make this easier, I have
started to create a "svn_test__sandbox" class, with which a C test can
very easily set up a repository and a working copy, like the Python
tests do.  Then it can run the low-level functions that it is
unit-testing.

[[[
  test_read_write_tree_conflicts(const svn_test_opts_t *opts,
                                 apr_pool_t *pool)
  {
    svn_test__sandbox_t sbox;
    ...

    SVN_ERR(svn_test__sandbox_create(&sbox, "read_write_tree_conflicts",
                                     opts, pool));
    SVN_ERR(svn_wc__db_op_set_tree_conflict(sbox.wc_ctx->db, child_abspath,
                                            conflict, pool));
    ...
  }
]]]

That may not be the purist way to perform unit testing, but it is very
practical.  Many of the libsvn_wc tests are already using it.

I intend that it should have a set of utility functions like the Python
test suite's "sbox.simple_update()" etc.  An example of this is
implemented in subversion/tests/libsvn_wc/op-depth-test.c, and used like
this:

  {
    svn_test__sandbox_t b;
  
    SVN_ERR(svn_test__sandbox_create(&b, "del_replace_not_present",
                                     opts, pool));
    SVN_ERR(wc_mkdir(&b, "A"));
    SVN_ERR(wc_mkdir(&b, "A/B"));
    SVN_ERR(wc_mkdir(&b, "A/B/X"));
    SVN_ERR(wc_mkdir(&b, "A/B/Y"));
    SVN_ERR(wc_mkdir(&b, "A/B/Z"));
    SVN_ERR(wc_commit(&b, ""));
    ...
  }


We can move it outside of tests/libsvn_wc to be available to the other
library tests as soon as we think it's useful enough.

I'm open to any feedback.

- Julian