build: restore running tests on Travis

Restore running tests on Travis once the ccache is populated.

PR-URL: #26720
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

.travis.yml

@@ -31,4 +31,4 @@ matrix:
         - ./configure
         - make -j2 V=
       script:
-        - # PARALLEL_ARGS='--flaky-tests=skip' make -j1 test
+        - PARALLEL_ARGS='--flaky-tests=skip' make -j1 test