Gotestsum (#22826)

* disable coverage

This reduces runtime of the server test suite from ~30m to ~10m, and as far as I can see: we discarded the coverage output anyway.

* allow morph 60s to migrate when running tests

* scripts/test.sh: drop COVERMODE

Stop generating coverage data when running unit tests. It's likely we'll want this data back at some point, but for now it's unused and removing simplifies invoking tests for developers.

* scripts/test.sh: remove cleanup steps

* scripts/test.sh: drop TESTS parameter

* scripts/test.sh: drop TESTFLAGS parameter

* switch to gotestsum

server/.gitignore

@@ -113,6 +113,7 @@ cprofile.out
 *.test
 webapp/coverage
 /report.xml
+/gotestsum.json
 
 .agignore
 .ctags