automake 1.13 test changes

In automake 1.13, the default is now to running tests in "make check" in parallel, redirect output to trace files and produce a report.

This causes problems for us when we have multiple tests in the same directory (e.g., examples/c), because multiple processes are forked in parallel and all of them expect exclusive access to WT_TEST.

There is a way to revert to the previous behavior, via a flag called "serial-tests". Unfortunately, this flag was only added to automake 1.12, so if we turn it on we can't support earlier versions. Our test machine bengal is up-to-date with the latest version of CentOS and has 1.11.

The choices are:

1. install the latest automake on all machines we care about, and require it for anyone building WT from the git tree;
2. find a way to turn on serial-tests if the automake version is 1.13, which turns out to be a pain because automake scans configure.ac from perl code without executing the logic; or
3. update our tests to work if run in parallel (e.g., have each example use a different directory, possibly via WIREDTIGER_HOME).