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

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

Change-Id: I957ad2e7c1e3b8d0ac252d38ab3bc46fbe3eabc7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>