tests/sprintf: Enable picolibc floating point printf for new test

Make sure we test floating point output too.

Signed-off-by: Keith Packard <keithp@keithp.com>
diff --git a/tests/lib/sprintf/prj_picolibc_new.conf b/tests/lib/sprintf/prj_picolibc_new.conf
index b6d3df3..458e98c 100644
--- a/tests/lib/sprintf/prj_picolibc_new.conf
+++ b/tests/lib/sprintf/prj_picolibc_new.conf
@@ -1,6 +1,7 @@
 CONFIG_ZTEST=y
 CONFIG_ZTEST_NEW_API=y
 CONFIG_PICOLIBC=y
+CONFIG_PICOLIBC_IO_FLOAT=y
 CONFIG_STDOUT_CONSOLE=n
 CONFIG_TEST_USERSPACE=y
 CONFIG_ZTEST_FATAL_HOOK=y