lib/Testing: add helper module to access test flags

Notably, this lets me plumb +v for verbose test output.

lib/Testing.bsv

@@ -36,6 +36,21 @@ module mkCycleCounter(CycleCounter);
    method all = total._read;
 endmodule
 
+interface TestFlags;
+   method Bool verbose();
+endinterface
+
+module mkTestFlags(TestFlags);
+   Wire#(Bool) verbose_val <- mkBypassWire();
+
+   rule every;
+      let v <- $test$plusargs("v");
+      verbose_val <= v;
+   endrule
+
+   method verbose = verbose_val;
+endmodule
+
 // mkTest runs the given test, printing status text before and after
 // the run. Tests can be nested.
 function Stmt mkTest(String name, Stmt test);