As a Meson developer it's often frustrating to have a single functional
test with a regression. These tests can be awkward to reproduce,
especially when they make use of a test.json file. This script provides
a simmple interface to call functional tests 1 at a time, regardless of
whether they use a test.json or not. If they do use a test.json, and
have a matrix, then the `--subtest` option can be used to select spcific
combinations, for example:
```sh
./run_single_test.py "test cases/frameworks/15 llvm" --subtest 1
```
will run only the second (zero indexed of course) subtest from the llvm
test cases.
This is not a super elegent script, but this is super useful.
Jon Turney