"un-skip" more skipped tests

* version_compile_os can be "linux-systemd", not equal to "Linux"
* main.no-threads forces no-threads scheduler, a check whether it
  has one_thread_per_connection is guaranteed to fail.

mysql-test/include/linux.inc

@@ -1,4 +1,4 @@
-if (`select convert(@@version_compile_os using latin1) LIKE 'Linux' = 0`)
+if (`select @@version_compile_os not LIKE 'Linux%'`)
 {
   skip Need Linux;
 }

mysql-test/main/no-threads.test

@@ -2,7 +2,6 @@
 # an additional util connection and other statistics data
 -- source include/no_view_protocol.inc
 
---source include/one_thread_per_connection.inc
 #
 # Test the --thread-handler=no-threads option
 #

mysql-test/suite/plugins/t/show_all_plugins.test

@@ -1,7 +1,7 @@
+--source include/have_udf.inc
 if (!$DIALOG_EXAMPLES_SO) { skip requires dialog_examples.so; }
 if (!$HA_EXAMPLE_SO) { skip requires ha_examples.so; }
 if (!$LIBDAEMON_EXAMPLE_SO) { skip requires libdaemon_examples.so; }
-if (!$UDF_EXAMPLE_SO) { skip requires udf_example.so; }
 if (!$EXAMPLE_KEY_MANAGEMENT_SO) { skip requires example_key_management.so; }
 if (`SELECT VERSION() LIKE '%embedded%'`) { skip Disabled for embedded until MDEV-8664 is resolved; }