`exclude` supports `./tests` but not `tests` (with the default `.` target)

[huyz]

Describe the bug
It took me way too long to figure out why exclude: tests didn't work. It seems that counter-intuitively tests doesn't work to exclude directories when the default target is ., presumably because Bandit sees the directory as ./tests and doesn't recognize that tests and ./tests refer to the same directory..

To Reproduce
Steps to reproduce the behavior:

1. Have a tests/ subdirectory of files with some bandit issues.
2. Specify exclude: tests
3. Leave default target as .

Expected behavior
Bandit action doesn't report warnings for any files under tests/

Actual behavior
Bandit scans the tests and reports warnings.