examples: factor the test suite.

examples: factor the test suite.

	* examples/mfcalc/test, examples/calc++/test: Extract the
	common bits into...
	* examples/test: here.
	(cwd): New.
	Use it to avoid a race on the temporary directory.
	Reported by Jim Meyering.
	* examples/mfcalc/test, examples/calc++/test: Rename into...
	* examples/mfcalc/mfcalc.test, examples/calc++/calc++.test: these.
	* examples/calc++/local.mk, examples/mfcalc/local.mk,
	* examples/local.mk: Adjust.