Fix path references in the benchmarking and profiling tox configurrations

tox.ini

@@ -85,9 +85,10 @@ whitelist_externals = sh
 deps =
     -r{toxinidir}/requirements_dev.txt
     -r{toxinidir}/requirements_continuous_integration.txt
+changedir = {toxinidir}    
 commands =
     python --version
-    python benchmarks/benchmark_all_examples.py
+    python {toxinidir}/benchmarks/benchmark_all_examples.py
 
 [testenv:profiling]
 ; use python3.6 because of incompatibility under Windows of the pycairo installed through conda for py37
@@ -110,8 +111,9 @@ conda_channels :
 deps =
     -r{toxinidir}/requirements_dev.txt
     -r{toxinidir}/requirements_continuous_integration.txt
+changedir = {toxinidir}    
 commands =
-    python benchmarks/profile_all_examples.py
+    python {toxinidir}/benchmarks/profile_all_examples.py
 
 [testenv:examples]
 ; TODO should examples be run with py36 and py37??