Fix up .gitignore and cleanup actions in some src/test/ subdirectories.

Fix up .gitignore and cleanup actions in some src/test/ subdirectories.

examples/, locale/, and thread/ lacked .gitignore files and were also
not connected up to top-level "make clean" etc.  This had escaped notice
because none of those directories are built in normal scenarios.  Still,
they have working Makefiles, so if someone does a "make" in one of these
directories it would be good if (a) git doesn't bleat about the product
files and (b) cleaning up removes them.

This is a longstanding oversight, but since this behavior is probably
only of interest to developers, there seems no need for back-patching.

Michael Paquier and Tom Lane