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/unsorted/stopfirst.sby

@@ -0,0 +1,16 @@
+[options]
+mode bmc
+expect fail
+
+[engines]
+btor btormc
+
+[script]
+read -sv test.sv
+prep -top test
+
+[file test.sv]
+module test(input foo);
+always @* assert(foo);
+always @* assert(!foo);
+endmodule