Implement generic CI step "may_fail" option

There might be various occasions where a CI step should be attempted to
be executed, despite a chance of failing.  For instance, we might want
to try executing an instable Iperf test, and still mark the CI as
passing if it fails.

This commit introduces a generic "may_fail" option that will mark any CI
test as succeeded, despite having failed.

Ideally, we would print a warning in the HTML code, but this is not
possible because each step emits its own HTML code, instead of the main
loop handling this. We can improve this in the future.

ci-scripts/tests/test-runner/test.xml

@@ -27,6 +27,8 @@
         <htmlTabIcon>wrench</htmlTabIcon>
         <TestCaseRequestedList>
  000001
+ 000011
+ 000012
  000002
  000003
  000004
@@ -41,6 +43,23 @@
                 <command_fail>yes</command_fail>
         </testCase>
 
+        <testCase id="000011">
+                <class>Custom_Command</class>
+                <desc>This may fail</desc>
+                <may_fail>true</may_fail>
+                <node>localhost</node>
+                <command>false</command>
+                <command_fail>yes</command_fail>
+        </testCase>
+
+        <testCase id="000012">
+                <class>Custom_Command</class>
+                <desc>This is still executed</desc>
+                <node>localhost</node>
+                <command>true</command>
+                <command_fail>yes</command_fail>
+        </testCase>
+
         <testCase id="000002">
                 <class>Custom_Command</class>
                 <desc>This should fail</desc>