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/keepgoing/keepgoing_smtc.sby

@@ -0,0 +1,19 @@
+[options]
+mode bmc
+expect fail
+
+[engines]
+smtbmc --keep-going boolector -- --smtc src/extra.smtc
+
+[script]
+read -sv keepgoing_same_step.sv
+prep -top test
+
+[files]
+keepgoing_same_step.sv
+
+[file extra.smtc]
+state 2
+assert false
+always
+assert (distinct [counter] #b00000111)