Avoid void* pointer arithmetic

* tests/tsexp.c (check_extract_param): Cast void* pointers to char*
before doing arithmetics.

GCC was complaining:
tsexp.c: In function ‘check_extract_param’:
tsexp.c:938:44: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
tsexp.c:944:46: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
tsexp.c:955:44: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
tsexp.c:961:46: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]

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