Add test for copying directories

As per https://stackoverflow.com/a/54950959, `os.path.basename()` returns an empty string if the string ends with a trailing slash.  This means that the target implied by `dir/` differs from an explicit target of `dir/`, and changes the behaviour to copy files to the root `src` directory instead.

tests/links/symlink.py

@@ -1,4 +1,3 @@
-import os
 from pathlib import Path
 import sys
 
@@ -13,6 +12,7 @@ def main():
                 assert(local_contents.strip() == 'log foo')
         else:
             assert(srcfile.is_symlink() == (task == "link"))
+            assert(srcfile.name != "script.ys")
 
 if __name__ == "__main__":
     main()

tests/links/symlink.sby

@@ -1,6 +1,8 @@
 [tasks]
 link
 copy
+dir_implicit: dir
+dir_explicit: dir
 
 [options]
 mode prep
@@ -15,7 +17,9 @@ script dir/script.ys
 [files]
 ../../docs/examples/demos/picorv32.v
 prv32fmcmp.v
-dir
+~dir: dir
+dir_implicit: dir/
+dir_explicit: dir/ dir/
 
 [file heredoc]
 log foo