twister: Better manage gcov sym link

In twister coverage runs we create the gcov sym link to llvm-cov
executable. This code runs multiple times which causes errors with the
current implementation since creating the symlink will fail; then
copying will also fail because we're using the symlink (self copy the
file).

Add a helper function to `try_making_symlink` which will handle the
following cases:
1. If the symlink already exists and is correct, do nothing.
2. If the symlink exists but points to the wrong file, replace it.
3. If the symlink doesn't exist create it.
4. If creating a symlink fails, just copy the file.

Signed-off-by: Yuval Peress <peress@google.com>