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/multi_assert.sby

@@ -0,0 +1,24 @@
+[tasks]
+btormc
+pono
+
+[options]
+mode bmc
+depth 5
+expect fail
+
+[engines]
+btormc: btor btormc
+pono: btor pono
+
+[script]
+read -sv multi_assert.v
+prep -top test
+
+[file multi_assert.v]
+module test();
+always @* begin
+assert (1);
+assert (0);
+end
+endmodule