Use unw_context_t instead of ucontext_t in tests/Gtest-trace.c and tests/Gtest-bt.c

At least on ARM unw_context_t and ucontext_t are not the same types, so
use unw_context_t.

See also commit 24112f6d9b87554fe18b1ca0f939f30c76ac38fa ("Fix some test
failures on x86_64 on distros with small default stacks.")

Gtest-trace.c: In function 'do_backtrace':
Gtest-trace.c:66:3: warning: initialization from incompatible pointer type [enabled by default]
Gtest-trace.c:67:3: warning: passing argument 2 of '_Uarm_init_local' from incompatible pointer type [enabled by default]
../include/libunwind-common.h:239:1: note: expected 'struct unw_context_t *' but argument is of type 'struct ucontext_t *'

Gtest-bt.c: In function 'do_backtrace':
Gtest-bt.c:65:3: warning: initialization from incompatible pointer type [enabled by default]
Gtest-bt.c:66:3: warning: passing argument 2 of '_Uarm_init_local' from incompatible pointer type [enabled by default]
../include/libunwind-common.h:239:1: note: expected 'struct unw_context_t *' but argument is of type 'struct ucontext_t *'
2 files changed