Update test expectations based on latest mongo gem

This commit updates the test expectation for an updated message string. The same
behavior is observbed, the message has just been updated.

This commit also updates construction of a test mongodb client with a newly
required value for auth_source when building a simple client.

Author: donoghuc

instrumentation/mongo/test/opentelemetry/instrumentation/mongo/subscriber_test.rb

@@ -380,7 +380,7 @@ module MongoTraceTest
       _(span.events[0].name).must_equal 'exception'
       _(span.events[0].timestamp).must_be_kind_of Integer
       _(span.events[0].attributes['exception.type']).must_equal 'CommandFailed'
-      _(span.events[0].attributes['exception.message']).must_equal 'ns not found (26)'
+      _(span.events[0].attributes['exception.message']).must_equal '[26:NamespaceNotFound]: ns not found'
     end
 
     describe 'that triggers #failed before #started' do
@@ -402,7 +402,8 @@ module MongoTraceTest
         database: TestHelper.database,
         auth_mech: :plain,
         user: 'plain_user',
-        password: 'plain_pass'
+        password: 'plain_pass',
+        auth_source: '$external'
       }
     end