tests/shell: special handle base path starting with "./"

tests/shell: special handle base path starting with "./"

When we auto detect the tests with `tests/shell/run-tests.sh -L`, then
commonly the NFT_TEST_BASEDIR starts with a redundant "./". That's a bit
ugly.

Instead, special handle that case and remove the prefix. The effect is
that `tests/shell/run-tests.sh -L` shows

  tests/shell/testcases/bitwise/0040mark_binop_0

instead of

  ./tests/shell/testcases/bitwise/0040mark_binop_0

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>