Move integration tests to tests/ directory (#973)

Reviewed-on: https://gitea.com/gitea/tea/pulls/973

tests/README.md

@@ -0,0 +1,10 @@
+This directory contains integration tests that exercise tea against external services or external executables.
+
+- Unit tests stay next to the packages they cover.
+- Integration tests live under `tests/` so they can be run separately.
+
+Common targets:
+
+- `make unit-test`
+- `make integration-test`
+- `make test`