Test datadir in conf file exists
Test datadir in conf file exists

Summary:
If a custom data directory is specified using -datadir argument, its
existence is checked before the conf file is loaded. But if the conf
file then specifies a different non-existent datadir, that isn't tested,
and results in esoteric errors

Backport of core PR11829

Test Plan:
Run `bitcoind -datadir=<dir>` in the following conditions:
1. <dir> exists, check it works
2. <dir> does not exist, check it fails
Now create a config file with the line `datadir=<conf_dir>` and run
`bitcoind -datadir=<dir> -conf=<path_to_config_file>`:
3. <dir> exists, but not <config_dir>, check it works
4. <dir> does not exist, but <config_dir> exists, check it fails
All the failure should display an error in the console: "specified data
directory (...) does not exist"

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2277

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