[pitivi] docs: Update HACKING with how we run unittests

[Alexandru Băluț <alexbalut src gnome org>]

commit cacad15bbde4fb95307fd9bb10e79b0718d95e38
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Fri Jan 25 22:32:48 2019 +0100

    docs: Update HACKING with how we run unittests

 docs/HACKING.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/docs/HACKING.md b/docs/HACKING.md
index 9bd292ad..aba11c93 100644
--- a/docs/HACKING.md
+++ b/docs/HACKING.md
@@ -66,8 +66,8 @@ in `pitivi-dev/pitivi/mesonbuild`
 Now that you are in the development environment, try running the
 [unittests](Testing.md):
 ```
-(ptv-flatpak) $ ninja -C mesonbuild/ test
-Running in sandbox: ninja -C mesonbuild/ test
+(ptv-flatpak) $ ptvtests
+Running in sandbox: gst-validate-launcher .../pitivi/tests/ptv_testsuite.py
 ```
 
 Hack away, and check the effect of your changes by simply running: