depcomp: fix bugs in tests and in the depcomp script

depcomp: fix bugs in tests and in the depcomp script

* lib/depcomp (gcc): Remove duplicated `-e' from sed invocation.
* tests/depcomp10.test: Make it executable.  Fix a blunder that
has left part of an intended comment not prefixed by `#', thus
causing shell syntax errors.  In this same comment, break a
too-long reported error message on multiple lines, for clarity.
Add reference to the relevant bug report.  Add a comment which
explains why the test result 'skipped' if the first "make" call
fails.  Add other useful comments.
* tests/depcomp9.test: Slightly improve comments.