utests: define python interpreter via cmake variable
utests: define python interpreter via cmake variable

The reason for this fix is in commit
5b64170ef5e3e78d038186fb1132b11a8fec308e.

Signed-off-by: Boqun Feng <4e4905b5cb0d447358c6a2b2a203fe3f2ade002e@gmail.com>
Reviewed-by: Igor Gnatenko <c869b0bfe2cf1da26c2efd33f6d6d74ddcbbdd2b@gmail.com>
Reviewed-by: Zhigang Gong <e04a7b9b70b1e4c6318cf117dcd1a9056e14b97a@linux.intel.com>

wdv4758h/beignet,freedesktop-unofficial-mirror/beignet,ignatenkobrain/beignet,zhenyw/beignet,wdv4758h/beignet,freedesktop-unofficial-mirror/beignet,zhenyw/beignet,ignatenkobrain/beignet,wdv4758h/beignet,zhenyw/beignet,wdv4758h/beignet,freedesktop-unofficial-mirror/beignet,ignatenkobrain/beignet,wdv4758h/beignet,zhenyw/beignet,ignatenkobrain/beignet,ignatenkobrain/beignet,zhenyw/beignet,freedesktop-unofficial-mirror/beignet