travis-ci: fine tune the use of 'set -x' in 'ci/*' scripts

travis-ci: fine tune the use of 'set -x' in 'ci/*' scripts

The change in commit 4f2636667 (travis-ci: use 'set -x' in 'ci/*'
scripts for extra tracing output, 2017-12-12) left a couple of rough
edges:

  - 'ci/run-linux32-build.sh' is executed in a Docker container and
    therefore doesn't source 'ci/lib-travisci.sh', which would enable
    tracing executed commands.  Enable 'set -x' in this script, too.

  - 'ci/print-test-failures.sh' iterates over all the files containing
    the exit codes of all the executed test scripts.  Since there are
    over 800 such files, the loop produces way too much noise with
    tracing executed commands enabled, so disable 'set -x' for this
    script.

  - 'ci/run-windows-build.sh' busily waits in a loop for the result of
    the Windows build, producing too much noise with tracing executed
    commands enabled as well.  Disable 'set -x' for the duration of
    that loop.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>