refactor: move e2e_test.go and its testdata to internal (#2934)

Move `e2e_test.go` and related test data to internal/integration.
- Change package to `integration_test` following Go's convention for
external test packages. This ensures e2e test only interact with the
exported APIs (which it is already doing).
- The `e2e` specific test data has been moved from
`internal/testdata/e2e` to `internal/integration/testdata/e2e`. This
adheres to the principle of co-location, keeping test data alongside the
tests that use it.
- Update testing doc and workflow to reflect changes.

The rest of testdata should also be moved to out of root into `internal`
and co-locate in the package of the test, will do that in a separate PR.

For #2842

Author: zhumin8

.github/workflows/librarian.yaml

@@ -62,7 +62,7 @@ jobs:
       - name: Build the test image
         run: |
           DOCKER_BUILDKIT=1 docker build \
-            -f ./testdata/e2e-test.Dockerfile \
+            -f ./internal/integration/testdata/e2e-test.Dockerfile \
             -t test-image:latest \
             .
       - name: Run end-to-end test

doc/testing.md

@@ -20,15 +20,15 @@ Setup:
 
 ```bash
 DOCKER_BUILDKIT=1 docker build \
-  -f ./testdata/e2e-test.Dockerfile \
+  -f ./internal/integration/testdata/e2e-test.Dockerfile \
   -t test-image:latest \
   .
 ```
 
 Usage:
 
 ```bash
-go test -tags e2e
+go test -tags e2e ./internal/integration/...
 ```
 
 ## Integration Tests

e2e_test.go renamed to internal/integration/e2e_test.go

@@ -14,7 +14,7 @@
 
 //go:build e2e
 
-package librarian
+package integration_test
 
 import (
 	"bytes"

testdata/e2e-test.Dockerfile renamed to internal/integration/testdata/e2e-test.Dockerfile

@@ -22,7 +22,7 @@ COPY go.sum .
 
 RUN go mod download
 
-COPY ./testdata/e2e_func.go .
+COPY ./internal/integration/testdata/e2e_func.go .
 
 RUN go build -o e2e_func .