Hoist up Node Gradle plugin, set TestTomcat baseDir, add hasContentType matcher, HTTP status consts

[mbland]

This is a redo of #3, which I couldn't get to trigger any GitHub Actions, even after merging #4.

Combining several small touch up changes in one:

Hoist Node Gradle plugin up to settings.gradle.kts

settings.gradle.kts seems a better place to specify the versions of Gradle plugins than in the project specific build.gradle.kts files.

Set TestTomcat baseDir explicitly

This is in preparation for possibly launching a TestTomcat per test method, instead of per test fixture/suite.

I found out that trying to dynamically create a baseDir for each instance led to the first instance's baseDir reappearing. I haven't been able to see clearly how this is happening, but it seems that:

1. The baseDir isn't strictly necessary in our case, since we're not running JSPs.
2. Using the same build/test-tomcat baseDir for all instances and having each instance delete it when finished isn't breaking anything.

Add hasContentType matcher, HTTP status consts

This matcher removes a lot of boilerplate from checking the Content-Type of a HttpResponse. It's based on the Hamcrest API with the help of these tutorials:

• https://hamcrest.org/JavaHamcrest/tutorial
• https://www.baeldung.com/hamcrest-custom-matchers

This could be generalized to handle any header. However, I want to keep it straightforward for now, as a good basic example of how to write a custom matcher.

Also took the opportunity to replace bare HTTP status codes (200, et. al.) with constants defined on jakarta.servlet.http.HttpServletResponse.

[mbland]

Nope, still not working. Let me see if a new branch will help. If all else fails, we'll see if #5 helps.