test-runner: Add ability to perform basic divisions and multiplications

We may need to compare monitor size to fractions and while it can be
done using inverse divisions, they were not supported as we didn't parse
floating point values.

So, just add support for both multiplications and divisions, so that we
can easily do stuff like `MONITOR_HEIGHT*3/4`.

Additions would be easy to support too if we don't care about operator
priorities, but that's out of scope for now.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3701>

src/tests/test-runner.c

@@ -429,22 +429,66 @@ test_case_check_xserver_stacking (TestCase *test,
 }
 
 static int
-maybe_divide (const char *str,
-              int         value)
+maybe_multiply (const char  *str,
+                int          value,
+                const char **out_str)
 {
-  if (strstr (str, "/") == str)
+  *out_str = str;
+
+  if (str[0] == '*')
     {
-      int divisor;
+      double multiplier;
 
       str += 1;
-      divisor = atoi (str);
+      multiplier = g_strtod (str, (char **) out_str);
 
-      value /= divisor;
+      value = round (multiplier * value);
     }
 
   return value;
 }
 
+static int
+maybe_divide (const char  *str,
+              int          value,
+              const char **out_str)
+{
+  *out_str = str;
+
+  if (str[0] == '/')
+    {
+      double divider;
+
+      str += 1;
+      divider = g_strtod (str, (char **) out_str);
+
+      value = round (value / divider);
+    }
+
+  return value;
+}
+
+static int
+maybe_do_math (const char  *str,
+               int          value,
+               const char **out_str)
+{
+  switch (str[0])
+    {
+    case '*':
+      value = maybe_multiply (str, value, &str);
+      break;
+    case '/':
+      value = maybe_divide (str, value, &str);
+      break;
+    default:
+      *out_str = str;
+      return value;
+    }
+
+  return maybe_do_math (str, value, out_str);
+}
+
 static int
 parse_window_size (MetaWindow *window,
                    const char *size_str)
@@ -462,13 +506,13 @@ parse_window_size (MetaWindow *window,
     {
       value = logical_monitor_layout.width;
       size_str += strlen ("MONITOR_WIDTH");
-      value = maybe_divide (size_str, value);
+      value = maybe_do_math (size_str, value, &size_str);
     }
   else if (strstr (size_str, "MONITOR_HEIGHT") == size_str)
     {
       value = logical_monitor_layout.height;
       size_str += strlen ("MONITOR_HEIGHT");
-      value = maybe_divide (size_str, value);
+      value = maybe_do_math (size_str, value, &size_str);
     }
   else
     {