pw_transfer: Make integration_test_server testonly, fix fx roller

Make the pw_transfer:integration_test_client GN target testonly
to fix the Fuchsia roller.

Bug: 312493408
Test:
1) Checkout this CL in fuchsia/third_party/pigweed/src
2) $ fx build

Change-Id: I7bce013f8afbfeb0002c4ace6b91973725eceea9
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/182019
Commit-Queue: Ben Lawson <benlawson@google.com>
Reviewed-by: Taylor Cramer <cramertj@google.com>
Pigweed-Auto-Submit: Ben Lawson <benlawson@google.com>
diff --git a/pw_transfer/BUILD.gn b/pw_transfer/BUILD.gn
index e4e15d8..58a8782 100644
--- a/pw_transfer/BUILD.gn
+++ b/pw_transfer/BUILD.gn
@@ -273,6 +273,7 @@
 }
 
 pw_executable("integration_test_client") {
+  testonly = pw_unit_test_TESTONLY
   sources = [ "integration_test/client.cc" ]
   deps = [
     ":client",