tests: allow running 'make check' with ASAN

* tests/t-secmem.c (main): Skip test if environment variable
GCRYPT_IN_ASAN_TEST is defined.
* tests/t-sexp.c (main): Do not initialize secmem if environment
variable GCRYPT_IN_ASAN_TEST is defined.

ASAN and mlock are incompatible, so add GCRYPT_IN_ASAN_TEST
environment variant for skipping tests failing as result.
This allows easier automation of ASAN checks.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>