Merge "stdio_test: improve the %b/%B tests." am: e2c25fa87f am: b6fb4c313c

Original change: https://android-review.googlesource.com/c/platform/bionic/+/2548495

Change-Id: Idbdf3bfae8ef826f7d5523456a2f672ddd482a70
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>