Fix typos in test comments. * tests/posixsubst-data.test, tests/posixsubst-extradist.test, tests/posixsubst-ldadd.test, tests/posixsubst-libraries.test, tests/posixsubst-ltlibraries.test, tests/posixsubst-programs.test, tests/posixsubst-scripts.test, tests/posixsubst-sources.test, tests/posixsubst-tests.test: Fix typos. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>