Add CI testing job on CentOS (!4)

Add GitLab Continuous Integration job named 'test' which runs the
GParted unit tests and distcheck.  Note that the job starts from a fresh
official CentOS Docker image so also has to rebuild GParted too.

So far this job fails on unit test test_BlockSpecial.  Fragment of the
CI job log:

    make  check-TESTS
    make[2]: Entering directory `/builds/mfleetwo/gparted/tests'
    make[3]: Entering directory `/builds/mfleetwo/gparted/tests'
    PASS: test_dummy
    FAIL: test_BlockSpecial
    PASS: test_PasswordRAMStore
    PASS: test_PipeCapture
    make[4]: Entering directory `/builds/mfleetwo/gparted/tests'
    make[4]: Nothing to be done for `all'.
    make[4]: Leaving directory `/builds/mfleetwo/gparted/tests'
    ============================================================================
    Testsuite summary for gparted 0.31.0-git
    ============================================================================
    # TOTAL: 4
    # PASS:  3
    # SKIP:  0
    # XFAIL: 0
    # FAIL:  1
    # XPASS: 0
    # ERROR: 0
    ============================================================================
    See tests/test-suite.log
    Please report to https://bugzilla.gnome.org/enter_bug.cgi?product=gparted
    ============================================================================

Closes !4 - Add GitLab CI jobs to build and test GParted

.gitlab-ci.yml

@@ -5,10 +5,11 @@ before_script:
   - yum update -y
   - yum install -y which gnome-common gnome-doc-utils glib2-devel intltool
                    gcc-c++ libuuid-devel parted-devel gtkmm24-devel make
-                   polkit
+                   polkit file
 
 stages:
   - build
+  - test
 
 # Check GParted can be built and installed.
 build:
@@ -17,3 +18,12 @@ build:
     - ./autogen.sh
     - make
     - make install
+
+# Check GParted unit tests and distcheck pass.
+test:
+  stage: test
+  script:
+    - ./autogen.sh
+    - make
+    - make check
+    - make distcheck