Linking to utest or munit breaks HTML5 target

[Herschel]

Describe the bug
If an OpenFL project happens to link to utest or munit, the application ends up being instantiated twice:

1. from the embed call in the HTML5 template:

   openfl/assets/templates/html5/template/index.html

   Line 51 in c114edd

   lime.embed ("::APP_FILE::", "openfl-content", ::WIN_WIDTH::, ::WIN_HEIGHT::, { parameters: {} });

2. from an embed call in the ApplicationMain template, which seems to be specifically for handling test runners:

   openfl/assets/templates/haxe/ApplicationMain.hx

   Lines 21 to 25 in c114edd

   	#if (js && html5)
   	#if (munit || utest)
   	lime.system.System.embed("::APP_FILE::", null, ::WIN_WIDTH::, ::WIN_HEIGHT::);
   	#end
   	#else

This causes havoc as the preloader ends up running twice, loading all assets twice but failing, etc.

To Reproduce
Steps to reproduce the behavior:

1. Unzip this minimal sample: utest-test.zip
2. haxelib install utest
3. openfl test html5
4. Observe in the browser console output that Running is traced twice.

Expected behavior
The application is only instantiated once, and Running would only print once.

One solution is to avoid linking in these libraries for a non-test build, but it's easy to end up transitively depending on them from other libraries. (In my case we linked to hmm to parse some library info, which ended up depending on utest, and then the HTML5 target suddenly broke.)

The conditional #if utest lines in ApplicationMain seem to assume that this is an actual test suite using a different HTML5 template that doesn't embed the application itself, such as https://github.com/joshtynjala/openfl-utest-sample. One solution might be for test runners to use a define such as -D test to determine if this is an actual test suite as opposed to checking #if (utest || munit). I didn't investigate further to see how feasible this might be.

OpenFL Targets
HTML5

[joshtynjala]

When I find some time, I intend to clean up this conditional so that tests work with the default html5 template.

I've wondered about that conditional for a number of years, and I just can't come up with a plausible reason why it is there. My best guess is that it's something related to older versions of munit/utest, or older versions of OpenFL/Lime, that has since changed.