Makes regular expression selection on collections key case insensitive

This issue was initially reported by @michaelgranzow-avi on #2296.

@airween made an initial attempt to provide a fixed at #2107; As a
consequence of the pull request review - provided by @victorhora,
@zimmerle, and @michaelgranzow-avi - @airween made a second attempt
at #2297. After reviewing by @martinhsv, @zimmerle, I have absorbed
the essential pieces from @airween patch into this one.

This patch differs from @airween's because @airween's patches were
partially working: Key exclusions with regex weren't covered, same
for anchored variables (e.g. ARGS). During the review, I have
highlighted the importance of having elementary test cases. A simple
test case on ARGS could spot the issue. Since that is an important
fix, I don't want to hold this for one more review cycle; therefore,
I am committing the fix myself.

Thank you all involved in the solution of this very own issue.

src/variables/variable.h

@@ -116,9 +116,9 @@ class KeyExclusion {
 class KeyExclusionRegex : public KeyExclusion {
  public:
     explicit KeyExclusionRegex(const Utils::Regex &re)
-        : m_re(re.pattern) { }
+        : m_re(re.pattern, true) { }
     explicit KeyExclusionRegex(const std::string &re)
-        : m_re(re) { }
+        : m_re(re, true) { }
 
     ~KeyExclusionRegex() override { }
 
@@ -595,7 +595,7 @@ class VariableDictElement : public Variable {
 class VariableRegex : public Variable {
  public:
     VariableRegex(const std::string &name, const std::string &regex)
-        :  m_r(regex),
+        :  m_r(regex, true),
         m_regex(regex),
         Variable(name + ":" + "regex(" + regex + ")") { }