net: reenable SRV tests with _ldap._tcp.google.com
TestLookupDotsWithRemoteSource and TestLookupGoogleSRV
were disabled because they look up the no-longer-present
SRV record for _xmpp-server._tcp.google.com.
Change the tests to look for _ldap._tcp.google.com and
reenable them.
Fixes #56708.
Change-Id: I26475fa3ff6fc008048a4e5f24f0e96ee12f655c
Reviewed-on: https://go-review.googlesource.com/c/go/+/453861
Run-TryBot: Damien Neil <dneil@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>