test: subsys: cpp: Fix stack overflow on Armv8R-64
test: subsys: cpp: Fix stack overflow on Armv8R-64

After doing some tests on Armv8R aarch64 board, the used stack memory
exceeds the stack size for about 64 bytes. Extending ZTEST_STACKSIZE
to 5120 can fix it.

Signed-off-by: Jaxson Han <70391990969cde8f4f4c008c74e601e9ed3959a1@arm.com>

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