unit_test::thread fixes:

* Fail on exception in unit_test::thread
* Add unit_test:🧵:detach()

beast/unit_test/thread.h

@@ -94,6 +94,12 @@ public:
         s_->propagate_abort();
     }
 
+    void
+    detach()
+    {
+        t_.detach();
+    }
+
     void
     swap (thread& other)
     {
@@ -112,6 +118,15 @@ private:
         catch (suite::abort_exception const&)
         {
         }
+        catch (std::exception const& e)
+        {
+            s_->fail ("unhandled exception: " +
+                std::string (e.what()));
+        }
+        catch (...)
+        {
+            s_->fail ("unhandled exception");
+        }
     }
 };