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/submod_props.sby

@@ -0,0 +1,33 @@
+[tasks]
+bmc
+cover
+flatten
+
+[options]
+bmc: mode bmc
+cover: mode cover
+flatten: mode bmc
+
+expect fail
+
+[engines]
+smtbmc boolector
+
+[script]
+read -sv test.sv
+prep -top top
+flatten: flatten
+
+[file test.sv]
+module test(input foo);
+always @* assert(foo);
+always @* assert(!foo);
+always @* cover(foo);
+always @* cover(!foo);
+endmodule
+
+module top();
+test test_i (
+.foo(1'b1)
+);
+endmodule