tests: never source test-defs.sh directly, source test-lib.sh instead

tests: never source test-defs.sh directly, source test-lib.sh instead

After the recent re-organization, sourcing 'test-defs.sh' directly might
not work well and cause spurious failures or other unexpected behaviours.
We should source 'test-lib.sh' instead, which contains not more direct
code execution (only definition of shell variables/functions, or sourcing
of other '*.sh' with the same property), is protected against multiple
inclusions, and sources 'test-defs.sh' automatically in in a proper way.

* t/testsuite-summary-count.sh, t/tap-summary.sh, t/tap-summary-color.sh,
t/testsuite-summary-color.sh: Source 'test-lib.sh', not 'test-defs.sh'.
* gen-testsuite-part: Likewise, in the generated wrapper scripts.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>