gdbsupport: record and print failed selftest names

gdbsupport: record and print failed selftest names

Since "maint selftest" now runs quite a lot of tests (especially in an
all-targets build), I thought it would be useful to print a summary at
the end of what failed.  So, implement that.

Print the summary before the "Ran %d unit tests, %zu failed\n" line, so
that that one remains the last line, and the gdb.gdb/unittest.exp
doesn't need to be changed.

The output looks like (if I force a failure in a test):

    (gdb) maint selftest
    ...
    Running selftest value_copy.
    Running selftest xml_escape_text.
    Running selftest xml_escape_text_append.

    Failures:
      aarch64-analyze-prologue

    Ran 4134 unit tests, 1 failed
    (gdb)

Change-Id: If3aaabdd6f8078d0e6e50e8d08f3e558ab85277e
Approved-By: Tom Tromey <tom@tromey.com>