DX_PROG_AUTOTEST: Modernize.

DX_PROG_AUTOTEST: Modernize.

This macro can use some rework to behave a bit more like other program
probing macros in this package.

In particular, if Autotest is not found, configure now prints

  checking for autotest... no

rather than "... false", and configure no longer prints the next line
"checking if autotest works..." in this case.

Additionally, extra code to support snippet/autotest.mk is moved to
a new macro, DX_PROG_AUTOTEST_AM which also defines the necessary
conditional.

Finally, this macro now has coverage in the test suite.