Fix: cannot stat tests/STUB.h No such file or directory (#707)

Fix: cannot stat tests/STUB.h No such file or directory (#707)

Since 2b5ebbe (PR #670), we have been seeing "random" build failures.

The tests/Stub.am generated by source-maintenance.sh has not included
the tests/STUB.h file for explicit distribution. The source file was
built and included only when seen as a dependency of the files using it.

When stubs are copied for use by the extra binaries from tools/
directory, there is a secondary side effect from "make -j":

* When the -j concurrency is small, tests/STUB.h gets copied during the
  first cycle, and parallel builds compiling other copied files succeed.

* When the -j concurrency is large, tests/STUB.h may be deferred to a
  later copy cycle, and the first actual compile task needing it fails
  with `cannot stat 'src/tests/STUB.h': No such file or directory`.

Add tests/STUB.h to src/Makefile.am EXTRA_DIST to restore the previous
distribution behavior (when STUB_SOURCE contained it explicitly).

Update the pinger source paths that were omitted in 2b5ebbe.