support/testing: fix ATF Vexpress test case

This test case currently fails to build with:

./build/juno/release/bl1/context_mgmt.o: In function `cm_prepare_el3_exit':
context_mgmt.c:(.text.cm_prepare_el3_exit+0x54): undefined reference to `cm_set_next_context'
context_mgmt.c:(.text.cm_prepare_el3_exit+0x54): relocation truncated to fit: R_AARCH64_JUMP26 against undefined symbol `cm_set_next_context'

This issue has been fixed upstream in commit
10c252c14b7f446c0b49ef1aafbd5d37804577dd, available since v1.3. So
while we bump, let's bump to the latest version of ATF, v1.5.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/64360659



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>