Refactor tests

Organize tests into subdirectories and use a new makefile that scans
.sby files and allows selecting tests by mode, engine, solver and/or
subdirectory. Automatically skips tests that use engines/solvers that
are not found in the PATH.

See `cd tests; make help` for a description of supported make targets.

tests/junit/junit_expect.sby

@@ -0,0 +1,16 @@
+[options]
+mode bmc
+depth 1
+expect fail,timeout
+
+[engines]
+smtbmc
+
+[script]
+read -formal foo.v
+prep -top foo
+
+[file foo.v]
+module foo;
+always_comb assert(1);
+endmodule