Move generic format/sprintf warn specs to shared examples

Also use the `complain` matcher instead of `ruby_exe`

Author: Earlopain

core/kernel/format_spec.rb

@@ -12,36 +12,4 @@
   it "is accessible as a module function" do
     Kernel.format("%s", "hello").should == "hello"
   end
-
-  describe "when $VERBOSE is true" do
-    it "warns if too many arguments are passed" do
-      code = <<~RUBY
-        $VERBOSE = true
-        format("test", 1)
-      RUBY
-
-      ruby_exe(code, args: "2>&1").should.include?("warning: too many arguments for format string")
-    end
-
-    it "does not warns if too many keyword arguments are passed" do
-      code = <<~RUBY
-        $VERBOSE = true
-        format("test %{test}", test: 1, unused: 2)
-      RUBY
-
-      ruby_exe(code, args: "2>&1").should_not.include?("warning")
-    end
-
-    ruby_bug "#20593", ""..."3.4" do
-      it "doesn't warns if keyword arguments are passed and none are used" do
-        code = <<~RUBY
-          $VERBOSE = true
-          format("test", test: 1)
-          format("test", {})
-        RUBY
-
-        ruby_exe(code, args: "2>&1").should_not.include?("warning")
-      end
-    end
-  end
 end

core/kernel/shared/sprintf.rb

@@ -1031,4 +1031,27 @@ def obj.to_str; end
   it "does not raise error when passed more arguments than needed" do
     sprintf("%s %d %c", "string", 2, "c", []).should == "string 2 c"
   end
+
+  describe "when $VERBOSE is true" do
+    it "warns if too many arguments are passed" do
+      -> {
+        format("test", 1)
+      }.should complain(/too many arguments for format string/, verbose: true)
+    end
+
+    it "does not warns if too many keyword arguments are passed" do
+      -> {
+        format("test %{test}", test: 1, unused: 2)
+      }.should_not complain(verbose: true)
+    end
+
+    ruby_bug "#20593", ""..."3.4" do
+      it "doesn't warns if keyword arguments are passed and none are used" do
+        -> {
+          format("test", test: 1)
+          format("test", {})
+        }.should_not complain(verbose: true)
+      end
+    end
+  end
 end