Write test timing information to the build tree when running test_runner.py

Summary: Timing information is is stored 'timing.json'. It is not generated when build and src tree point to the same location.

Test Plan:
Preconditions: make sure, that you do not build from src tree:
mkdir build && cd build
../configure
make check

--> Test1: Run single test and check if its timing is present in build directory
rm build/timing.json
build/test/functional/test_runner.py uptime.py
cat build/timing.json

--> Test2: Run another test, it should be inserted in timing file before the previous one
build/test/functional/test_runner.py abc-cmdline.py
cat build/timing.json

--> Test3: Run another test. A single entry should be added in the middle of the timming.json
--> Note: this commit preserves the behavior, where it is not possible to include
--> or exclude an instance of the test with additional parameters - if explicitly dpecified
---> it will only run the test instance without parameters
build/test/functional/test_runner.py txn_doublespend.py
cat build/timing.json

--> Test4: Check that timing file nothing is not present in src directory
find -name timing.json

--> Test5: Switch to building form src tree, timing.json should not be generated there
rm build/timing.json
./configure
make check
test/functional/test_runner.py uptime.py
find -name timing.json

Reviewers: matiu, deadalnix, O1 Bitcoin ABC, #bitcoin_abc, schancel

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Maniphest Tasks: T163

Differential Revision: https://reviews.bitcoinabc.org/D1116