travis: move example tests to a separate run (#3143)

The example tests were causing travis timeout.

.travis.yml

@@ -10,6 +10,8 @@ matrix:
     env: RUN386=1
   - go: 1.13.x
     env: GRPC_GO_RETRY=on
+  - go: 1.13.x
+    env: TESTEXAMPLES=1
   - go: 1.12.x
     env: GO111MODULE=on
   - go: 1.11.x
@@ -28,13 +30,13 @@ before_install:
 install:
   - try3() { eval "$*" || eval "$*" || eval "$*"; }
   - try3 'if [[ "${GO111MODULE}" = "on" ]]; then go mod download; else make testdeps; fi'
-  - if [[ "${GAE}" = 1 ]]; then source ./install_gae.sh; make testappenginedeps; fi
-  - if [[ "${VET}" = 1 ]]; then ./vet.sh -install; fi
+  - if [[ -n "${GAE}" ]]; then source ./install_gae.sh; make testappenginedeps; fi
+  - if [[ -n "${VET}" ]]; then ./vet.sh -install; fi
 
 script:
   - set -e
-  - if [[ "${VET}" = 1 ]]; then ./vet.sh; fi
-  - if [[ "${GAE}" = 1 ]]; then make testappengine; exit 0; fi
-  - if [[ "${RACE}" = 1 ]]; then make testrace; exit 0; fi
+  - if [[ -n "${TESTEXAMPLES}" ]]; then examples/examples_test.sh; exit 0; fi
+  - if [[ -n "${VET}" ]]; then ./vet.sh; fi
+  - if [[ -n "${GAE}" ]]; then make testappengine; exit 0; fi
+  - if [[ -n "${RACE}" ]]; then make testrace; exit 0; fi
   - make test
-  - examples/examples_test.sh