Merge "stdio_test: improve the %b/%B tests." am: e2c25fa87f am: b6fb4c313c am: 07928fd142 am: 8e20546138

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

Change-Id: I27eee2c3e23e655772cf3705eb6b49c4e58fda34
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>