debuggerd_test: find backtrace frame in all lines in CrasherTest

Kernel can use vsyscall for system calls. The vsyscall implementation in
the kernel gives one more depth in the backtrace. It leads to failures
on CrasherTest. This CL makes tests find a system call frame not only in
the first line but also in all lines on the backtrace.

Bug: 62600694
Test: passes all CrasherTests.
Change-Id: Ice383bb94db097e7e9a9e4f74d8fa5ecc528122a