credentials/xds: fix goroutine leak in testServer (#8699)

credentials/xds: fix goroutine leak in testServer (#8699)

Fixes #8694

This PR fixes a goroutine leak in `credentials/xds/xds_client_test.go`.

Previously, the `testServer` used standard `Send()` calls . If a test
timed out or failed before reading the expected value, the `testServer`
goroutine would block indefinitely on the channel, causing a leak.

Replaced blocking `Send` calls with `SendContext` in `handleConn`. This
ensures that if the test ends (canceling the context), the `testServer`
stops trying to send and exits its goroutine gracefully.

RELEASE NOTES: None