Avoid double output in fenv_test

This tests fork()s, so if there is still data in the stdout buffer on fork
it will print it again in the child process. This was happening in the
CheriBSD CI and caused the test to complain about malformed TAP output.

Reviewed By:	ngie
Differential Revision: https://reviews.freebsd.org/D28397

lib/msun/tests/fenv_test.c

@@ -539,6 +539,8 @@ test_feholdupdate(void)
 int
 main(void)
 {
+	/* Avoid double output after fork() */
+	setvbuf(stdout, NULL, _IONBF, 0);
 
 	printf("1..8\n");
 	init_exceptsets();