[gdb/testsuite] Use allocator_may_return_null=1 in two test-cases

Simon reported [1] that recent commit 06e967dbc9b ("[gdb/python] Throw
MemoryError in inferior.read_memory if malloc fails") introduced
AddressSanitizer allocation-size-too-big errors in the two test-cases
affected by this commit.

Fix this by suppressing the error in the two test-cases using
allocator_may_return_null=1.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

[1] https://sourceware.org/pipermail/gdb-patches/2024-April/208171.html
3 files changed