gdb.base/skip.exp: Use finish to exit functions

gdb.base/skip.exp: Use finish to exit functions

gdb.base/skip.exp was making use of a fixed number of step commands to
exit some functions.  This caused some problems when using clang to test
GDB, as GDB would need fewer steps to reach the desired spots.  For
instance, when testing in the section "step after disabling 3", the log
looks like this:

    Breakpoint 4, main () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:32
    32        x = baz ((bar (), foo ()));
    (gdb) step
    bar () at binutils-gdb/gdb/testsuite/gdb.base/skip1.c:21
    21        return 1;
    (gdb) PASS: gdb.base/skip.exp: step after disabling 3: step 1
    step
    foo () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:42
    42        return 0;
    (gdb) PASS: gdb.base/skip.exp: step after disabling 3: step 2
    step
    main () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:34
    34        test_skip_file_and_function ();
    (gdb) step
    test_skip_file_and_function () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:59
    59        test_skip ();
    (gdb) FAIL: gdb.base/skip.exp: step after disabling 3: step 3
    step
    test_skip () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:48
    48      }
    (gdb) PASS: gdb.base/skip.exp: step after disabling 3: step 4
    step
    test_skip_file_and_function () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:60
    60        skip1_test_skip_file_and_function ();
    (gdb) FAIL: gdb.base/skip.exp: step after disabling 3: step 5

This shows that the feature is working but because the inferior lands in
a different location, it registers as a failure.  Seeing as along with
this difference, there are also some differences that depend on gcc
versions (where gdb might stop back at line 32 before entering foo), it
would not be easy to test for this behavior using steps and analzing
where the inferior stops at each point. On the other hand, using
gdb_step_until is not feasible because we'd possibly gloss over stepping
into baz and rendering the whole test useless.  Instead, skip.exp now
uses finish to leave functions, synchronizing through compilers and
compiler versions.  Some test names were also changed to be a bit more
descriptive.  The new log looks like this, independently of compiler used:

    Breakpoint 4, main () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:32
    32        x = baz ((bar (), foo ()));
    (gdb) step
    bar () at binutils-gdb/gdb/testsuite/gdb.base/skip1.c:21
    21        return 1;
    (gdb) PASS: gdb.base/skip.exp: step after disabling 3: step into bar
    finish
    Run till exit from #0  bar () at binutils-gdb/gdb/testsuite/gdb.base/skip1.c:21
    main () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:32
    32        x = baz ((bar (), foo ()));
    Value returned is $2 = 1
    (gdb) PASS: gdb.base/skip.exp: step after disabling 3: return from bar
    step
    foo () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:42
    42        return 0;
    (gdb) PASS: gdb.base/skip.exp: step after disabling 3: step into foo
    finish
    Run till exit from #0  foo () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:42
    main () at binutils-gdb/gdb/testsuite/gdb.base/skip.c:32
    32        x = baz ((bar (), foo ()));
    Value returned is $3 = 0
    (gdb) PASS: gdb.base/skip.exp: step after disabling 3: Return from foo
    step
    34        test_skip_file_and_function ();
    (gdb) PASS: gdb.base/skip.exp: step after disabling 3: step and skip baz