tests: Disable unsupported tests in CentOS

tests/build-gating-pocl.fmf

@@ -7,8 +7,10 @@ adjust:
       and trigger != build
     enabled: false
 
-  - because: "PoCL is shipped with Fedora, not RHEL"
-    when: "distro == rhel"
+  - because: "PoCL is shipped with Fedora, not RHEL/CentOS"
+    when: >-
+      distro == rhel
+      or distro == centos
     enabled: false
 
 discover:

tests/llvm-test-suite/main.fmf

@@ -2,8 +2,10 @@
 summary: Run tests from llvm-test-suite package
 
 adjust:
-  - because: "llvm-test-suite is available for Fedora, not RHEL"
-    when: "distro == rhel"
+  - because: "llvm-test-suite is available for Fedora, not RHEL/CentOS"
+    when: >-
+      distro == rhel
+      or distro == centos
     enabled: false
 
 test: "$WITH_SCL ./test.sh"

tests/pocl/main.fmf

@@ -5,8 +5,10 @@
 summary: pocl
 
 adjust:
-  - because: "PoCL is shipped with Fedora, not RHEL"
-    when: "distro == rhel"
+  - because: "PoCL is shipped with Fedora, not RHEL/CentOS"
+    when: >-
+      distro == rhel
+      or distro == centos
     enabled: false
 
 # This test requires a special preparation, see `/tests/tests-pocl` plan. As such,