gdb/testsuite: use build_executable in gdb.server/stop-reply-no-thread.exp

This test uses prepare_for_testing, then does a clean_restart for each
test configuration.  prepare_for_testing does a build_executable plus a
clean_restart.  So the clean_restart inside prepare_for_testing is done
for nothing.

Change prepare_for_testing to just build_executable to avoid the
unnecessary clean_restart.

gdb/testsuite/ChangeLog:

	* gdb.server/stop-reply-no-thread.exp: Use build_executable
	instead of prepare_for_testing.

Change-Id: I8b2a2e90353c57c39c49a3665083331b4882fdd0

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2021-01-04  Simon Marchi  <simon.marchi@efficios.com>
+
+	* gdb.server/stop-reply-no-thread.exp: Use build_executable
+	instead of prepare_for_testing.
+
 2021-01-04  Simon Marchi  <simon.marchi@efficios.com>
 
 	* gdb.server/solib-list.exp: Use clean_restart.

gdb/testsuite/gdb.server/stop-reply-no-thread.exp

@@ -28,7 +28,7 @@ if { [skip_gdbserver_tests] } {
 }
 
 standard_testfile
-if [prepare_for_testing "failed to prepare" $testfile $srcfile] {
+if { [build_executable "failed to prepare" $testfile $srcfile] == -1 } {
     return -1
 }