fix variables content used in tracker-testutils-3.0.pc

Without that patch nautilus meson build fails with

Run-time dependency tracker-testutils-3.0 found: YES 3.7.beta
Program /usr/lib64/tracker-3.0//trackertestutils/tracker-sandbox found: NO

test/automated/displayless/meson.build:3:18: ERROR: Program '/usr/lib64/tracker-3.0//trackertestutils/tracker-sandbox' not found or not executable

Fixes #428 (closed)

Signed-off-by: Tomasz Kłoczko kloczek@github.com