On CI, use a tmpfs for ZUUL_TEST_ROOT

Profiling shows git operations take significant time. They are done in
sub directories of a temporary directory optionally set via environment
variable ZUUL_TEST_ROOT.

Set ZUUL_TEST_ROOT at the jobs level.
When set, have test-setup to create a tmpfs.

Change-Id: I7923448aed1543593ace2cd9458b6c1b3ea60737

.zuul.yaml

@@ -199,12 +199,22 @@
               - playbooks/zuul-migrate/.*
             nodeset: ubuntu-xenial
             timeout: 3600  # 60 minutes
+            vars:
+                test_setup_environment:
+                    ZUUL_TEST_ROOT: /tmp/zuul-test
+                tox_environment:
+                    ZUUL_TEST_ROOT: /tmp/zuul-test
         - tox-py37:
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*
             timeout: 3600  # 60 minutes
             nodeset: ubuntu-bionic
+            vars:
+                test_setup_environment:
+                    ZUUL_TEST_ROOT: /tmp/zuul-test
+                tox_environment:
+                    ZUUL_TEST_ROOT: /tmp/zuul-test
         - zuul-build-dashboard
         - zuul-build-dashboard-multi-tenant
         - nodejs-npm-run-lint:
@@ -245,12 +255,22 @@
               - playbooks/zuul-migrate/.*
             nodeset: ubuntu-xenial
             timeout: 3600  # 60 minutes
+            vars:
+                test_setup_environment:
+                    ZUUL_TEST_ROOT: /tmp/zuul-test
+                tox_environment:
+                    ZUUL_TEST_ROOT: /tmp/zuul-test
         - tox-py37:
             irrelevant-files:
               - zuul/cmd/migrate.py
               - playbooks/zuul-migrate/.*
             timeout: 3600  # 60 minutes
             nodeset: ubuntu-bionic
+            vars:
+                test_setup_environment:
+                    ZUUL_TEST_ROOT: /tmp/zuul-test
+                tox_environment:
+                    ZUUL_TEST_ROOT: /tmp/zuul-test
         - zuul-build-dashboard
         - nodejs-npm-run-lint:
             vars:

tools/test-setup.sh

@@ -12,6 +12,12 @@ sudo service zookeeper stop
 DATADIR=$(sed -n -e 's/^dataDir=//p' /etc/zookeeper/conf/zoo.cfg)
 sudo mount -t tmpfs -o nodev,nosuid,size=500M none $DATADIR
 
+# Prepare a tmpfs for Zuul test root
+if [[ -n "${ZUUL_TEST_ROOT:-}" ]]; then
+    sudo mkdir -p "$ZUUL_TEST_ROOT"
+    sudo mount -t tmpfs -o noatime,nodev,nosuid,size=64M none "$ZUUL_TEST_ROOT"
+fi
+
 # Be sure mysql and zookeeper are started.
 sudo service mysql start
 sudo service postgresql start