fa811b0 qa: Normalize executable location (MarcoFalke)
Pull request description:
This removes the need to override the executable locations by just reading them from the config file. Beside making the code easier to read, running individual test on Windows is now possible by default (without providing further command line arguments).
Note: Of course, it is still possible to manually specify the location through the `BITCOIND` environment variable, e.g. `bitcoin-qt`.
Tree-SHA512: bee6d22246796242d747120ca18aaab089f73067de213c9111182561985c5912228a0b0f7f9eec025ecfdb44db031f15652f30d67c489d481c995bb3232a7ac7