Relative paths to `--report-file` can result in `FileNotFoundError`

[mahendrapaipuri]

When we run a simple regression test with setting --report-file on CLI as reframe --report-file=test-report.json -c ., we will end up with following error (pasting only relevant log lines):

[  PASSED  ] Ran 0/0 test case(s) from 0 check(s) (0 failure(s), 0 skipped)
[==========] Finished on Wed Apr  5 10:15:13 2023 
Run report saved in 'test-report.json'

ERROR: run session stopped: file not found error: [Errno 2] No such file or directory: ''
Log file(s) saved in '/tmp/rfm-dz27cl1s.log'

This is due to the fact that basedir here is being calculated on relative path of report_file and in the current case, basedir would be an empty string. Ensuring report_file to be an absolute path should fix the issue. This regression comes from this commit