Move tests

configure.ac

src/Makefile.am

src/tests/Makefile.am

src/tests/test-circle.cc

src/tests/test-gradient-horrible-logarithmic.cc

src/tests/test-gradient-horrible.cc

src/tests/test-gradient-identity.cc

src/tests/test-gradient-kinks.cc

src/tests/test-gradient-method-helper.hh

src/tests/test-gradient-method-nicefunction.hh

src/tests/test-gradient-parabola.cc

src/tests/test-gradient-sample-3d.cc

src/tests/test-gradient-sample-nonsmooth.cc

src/tests/test-gradient-sample-steep.cc

src/tests/test-gradient-sample-steep2.cc

src/tests/test-gradient-sample-verysteep.cc

src/tests/test-gradient-sample.cc

src/tests/test-gradient-sample2.cc

src/tests/test-gradient-trivial.cc

src/tests/test-minimise2.cc