GDB test coverage

[Yao Qi <qiyaoltc@gmail.com>]

Hi,
We had the doc on wiki about doing coverage tests to GDB,
https://sourceware.org/gdb/wiki/TestingGDB#Coverage_Testing
and I give it a try last week.  Get some interesting results to share
now,

http://people.linaro.org/~yao.qi/gdb/coverage/20161102/

In general, the coverage is better than I expected :-).  Some initial
analysis to the coverage data,

 - the coverage in linux-record.c is low, because we don't test many
   syscalls in gdb.reverse,
 - the coverage in *-lang.c is low.  Looks we are still lack of language
   related tests.
 - we don't test all the start options in gdb, see the coverage in
   main.c:captured_main_1
 - we don't test "set cp-abi", "set extension-language", "delete checkpoint"
   "detach checkpopint",
 - here is a list of functions are never used,
   main.c:get_gdb_program_name,
   corefile.c:read_stack,
   valarith.c:value_in,
 - linux_gdb_signal_from_target and linux_gdb_signal_to_target are not
   well covered,

I am sure we can still get more from these coverage results.  How do we
make full use of coverage tests?  What I can think of are
 - we can find some never-used functions, and remove some of them,
 - add more test cases to address some low-coverage areas,

-- 
Yao (齐尧)