bootstrap.conf: exercise more tests from Gnulib

bootstrap.conf: exercise more tests from Gnulib

This fixes the build failure with -Werror:

  configure:53786: gcc -o conftest -O0 -Wall -Werror -g3   conftest.c  -lev >&5
  conftest.c:412: error: "GNULIB_STRERROR" redefined [-Werror]
    412 | #define GNULIB_STRERROR 1
        |
  conftest.c:305: note: this is the location of the previous definition
    305 | #define GNULIB_STRERROR IN_GNUTLS_GNULIB_TESTS
        |
  cc1: all warnings being treated as errors

as well as improves code coverage.

Suggested by Bruno Haible in:
<https://lists.gnu.org/archive/html/bug-gnulib/2020-10/msg00148.html>.

Signed-off-by: Daiki Ueno <ueno@gnu.org>