lib/crypto: tests: Migrate Curve25519 self-test to KUnit

lib/crypto: tests: Migrate Curve25519 self-test to KUnit

Move the Curve25519 test from an ad-hoc self-test to a KUnit test.

Generally keep the same test logic for now, just translated to KUnit.
There's one exception, which is that I dropped the incomplete test of
curve25519_generic().  The approach I'm taking to cover the different
implementations with the KUnit tests is to just rely on booting kernels
in QEMU with different '-cpu' options, rather than try to make the tests
(incompletely) test multiple implementations on one CPU.  This way, both
the test and the library API are simpler.

This commit makes the file lib/crypto/curve25519.c no longer needed, as
its only purpose was to call the self-test.  However, keep it for now,
since a later commit will add code to it again.

Temporarily omit the default value of CRYPTO_SELFTESTS that the other
lib/crypto/ KUnit tests have.  It would cause a recursive kconfig
dependency, since the Curve25519 code is still entangled with CRYPTO.  A
later commit will fix that.

Link: https://lore.kernel.org/r/20250906213523.84915-8-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>