Check test metadata sanity during discover step

[psss]

When the path key contains an invalid directory the problem is revealed only during the test execution (and can be quite hard to investigate). We should make sure that path exists for each tests identified during the discover step and warn user early.

[idorax]

Hi @psss, would you please paste the fmf file having invalid path? I'll give it a try and help to fix it.

Currently I failed to reproduce it, e.g.

$ tree tests
tests
└── BADPATH
    ├── main.fmf
    └── test.sh

2 directories, 2 files
$ cat tests/BADPATH/main.fmf
summary: Concise summary describing what the test does
path: /BADPATH
test: ./test.sh
framework: beakerlib
$ tmt -vvv run
/var/tmp/tmt/run-002
Found 1 plan.

/plans/foo
summary: Basic smoke test
    discover
        how: fmf
        order: 50
        directory: /home/huanli/dev/1841/foo
        summary: 1 test selected
            /tests/BADPATH
    provision
        how: local
        order: 50
        distro: Fedora Linux 36 (Thirty Six)
        kernel: 6.1.14-100.fc36.x86_64
        summary: 1 guest provisioned
    prepare
        how: install
        summary: Install required packages
        name: requires
        order: 70
        package: 1 package requested
            beakerlib
            cmd: /bin/bash -c 'rpm -q --whatprovides beakerlib || sudo dnf install -y beakerlib'
            out: beakerlib-1.29.3-1.fc36.noarch
    
        summary: 1 preparation applied
    execute
        how: tmt
        order: 50
        exit-first: False
            test: Concise summary describing what the test does
    finish
        prune: Prune plan workdir '/var/tmp/tmt/run-002/plans/foo'.
        summary: 0 tasks completed
Failed to write '/var/tmp/tmt/run-002/plans/foo/discover/default-0/tests/BADPATH/tmt-test-wrapper.sh'.
[Errno 2] No such file or directory: '/var/tmp/tmt/run-002/plans/foo/discover/default-0/tests/BADPATH/tmt-test-wrapper.sh'
$ 

[psss]

Actually, you have reproduced the issue well: During the test execution user gets:

Failed to write '/var/tmp/tmt/run-002/plans/foo/discover/default-0/tests/BADPATH/tmt-test-wrapper.sh'.
[Errno 2] No such file or directory: '/var/tmp/tmt/run-002/plans/foo/discover/default-0/tests/BADPATH/tmt-test-wrapper.sh'

Instead, it would be nice to give a clear warning during the discover steps that path: /BADPATH is invalid.