TomcatReactiveWebServerFactory's getTomcatWebServer is described as
being a hook point the can be overrideden to return a different
TomcatWebServer or to apply additional processing to the Tomcat
server. Previously, this did not work as the method was not called
to create the TomcatWebServer instance. Instead, TomcatWebServer was
instantiated and returned directly.

This commit updates TomcatReactiveWebServerFactory to call
getTomcatWebServer to create the TomcatWebServer instance. A test
has also been added to verify that the method is now used.

Fixes gh-20386