lib/crypto: tests: Enable Curve25519 test when CRYPTO_SELFTESTS

Now that the Curve25519 library has been disentangled from CRYPTO,
adding CRYPTO_SELFTESTS as a default value of
CRYPTO_LIB_CURVE25519_KUNIT_TEST no longer causes a recursive kconfig
dependency.  Do this, which makes this option consistent with the other
crypto KUnit test options in the same file.

Link: https://lore.kernel.org/r/20250906213523.84915-12-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>