Test suite, step 2a: more unit tests; fix double wrapping after a GC

[balat]

Second step of the test suite: unit tests of pure server functions (no site, no request, no server). They found a bug in Wrap, fixed here.

Bug fix: double wrapping when the GC moves a shared value

Wrap.wrap traverses the value with a hash table indexed by block addresses. A block moved by a minor collection gets a second index; when the table is rehashed, the two entries are merged, keeping the largest index, whose substitution was still empty. A marked value shared inside the wrapped value, visited once before a collection and once after, was therefore wrapped again: the wrapper function ran twice (e.g. the data of Eliom_lazy values, which compute URLs, was computed twice) and the sharing was lost on the client.

The fix keeps, for the retained index, what was computed for the dropped one (on_merge callbacks of the table). The new test wrap / minor collections fails on every run without it.

Tests

• test_cookies: JSON serialisation of cookie sets sent to client processes; hashing of cookie values (with a known SHA-256 vector); session identifiers; names of state cookies built by make_full_cookie_name and parsed back by get_state_cookies (now exported in the internal part of common.server.mli) for every kind of state, hierarchy, security and cookie level; scope hierarchies created twice.
• test_wrap: replacement of marked values, sharing, cycles, nested wrapping, large values, minor collections during the traversal (with the second visit of a shared value on both the resizing and the rehashing paths of the table), a wrapper that raises; encode_eliom_data; string_escape, whose output is embedded in single-quoted strings of page scripts: no quote, markup or control character remains in clear, and the JavaScript literal gives the original bytes back, without legacy octal escapes.
• test_content: functional and DOM nodes and their identifiers, named and global elements, custom data, escaping, and escaping of client-sent parameter names and values in the error pages for wrong parameters.
• test_config: the options of the <eliom> element of the configuration file and their errors; each limit option sets its own limits and no other.

46 unit tests in total. Each file was checked against mutations of the code it covers.

Next steps

• 2b: tests with a site but no request (path services, absolute URLs, site-scope references, timeouts from the configuration, unregistered services).
• 3: test server harness and HTTP-level tests, including the responses of the registration modules. add_cache_header sends Cache-Control: max-age: N instead of max-age=N; it will be fixed there with a test.