trng: make init_trng(), exit_trng() and rand_bytes() test_mockable.
trng: make init_trng(), exit_trng() and rand_bytes() test_mockable.

Make these functions test_mockable so that crypto-related functions
that use them can be unit-tested.

BRANCH=nocturne
BUG=chromium:927095
TEST=make -j buildall

Change-Id: Ie59c232077fa14420ca9686409159173815f0d26
Signed-off-by: Yicheng Li <77a205905d8955597c069555e9569c30512f6bd8@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1652626
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec