Add tests to make sure setvbuf works.

tests/Makefile

@@ -14,6 +14,7 @@ EXPECT_BINS=\
 	setjmp \
 	signal \
 	stdio/all \
+	stdio/setvbuf \
 	stdio/freopen \
 	stdio/fwrite \
 	stdio/getc_unget \

tests/stdio/setvbuf.c

+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char ** argv) {
+	setvbuf(stdout, 0, _IONBF, 0);
+	FILE *f = fopen("stdio/stdio.in", "r");
+	setvbuf(f, 0, _IONBF, 0);
+	printf("%c\n", fgetc(f));
+	ungetc('H', f);
+	char *in = malloc(30);
+	printf("%s\n", fgets(in, 30, f));
+	printf("Hello\n");
+	return 0;
+}