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/check_output.py

@@ -0,0 +1,17 @@
+import re
+
+
+def line_ref(dir, filename, pattern):
+    with open(dir + "/src/" + filename) as file:
+        if isinstance(pattern, str):
+            pattern_re = re.compile(re.escape(pattern))
+        else:
+            pattern_re = pattern
+            pattern = pattern.pattern
+
+        for number, line in enumerate(file, 1):
+            if pattern_re.search(line):
+                # Needs to match source locations for both verilog frontends
+                return fr"{filename}:(?:{number}|\d+.\d+-{number}.\d+)"
+
+        raise RuntimeError("%s: pattern `%s` not found" % (filename, pattern))