Isolate static assets handler spec from others

Running `crystal spec` without a file argument essentially produces one
big program that combines every single spec file, their imports, and
the files that those imports themselves depend on. Most of the types
within this combined program will get ignored by the compiler due to a
lack of any calls to them from the spec files.

But for some types, partially the HTTP module ones, using them within
the spec files will suddenly make the compiler enable a bunch of
previously ignored code. And those code will suddenly require the
presence of additional types, constants, etc. This not only make it
annoying for getting the specs working but also makes it difficult to
isolate behaviors for testing.

The `static_assets_handler_spec.cr` causes this issue and so will be
marked as an isolated spec for now. In the future all of the tests
should be organized into independent groupings similar to how the
Crystal compiler splits their tests into std, compiler, primitives and
interpreter.

spec/http_server/handlers/static_assets_handler_spec.cr

@@ -1,4 +1,13 @@
-{% skip_file if compare_versions(Crystal::VERSION, "1.17.0-dev") < 0 %}
+# Due to the way that specs are handled this file cannot be run together with
+# everything else without causing a compile time error that'll be incredibly
+# annoying to resolve.
+#
+# TODO: Create different spec categories that can then be ran through make.
+#       An implementation of this can be seen with the tests for the Crystal compiler itself.
+#
+# For now run this with `crystal spec spec/http_server/handlers/static_assets_handler_spec.cr -Drunning_by_self`
+
+{% skip_file if compare_versions(Crystal::VERSION, "1.17.0-dev") < 0 || !flag?(:running_by_self) %}
 
 require "http"
 require "spectator"