Small check for semantic check

Staring with a simple pattern for checking for non-variables at left hand
side like [[ LHS == $value ]]. The file is supposed be amended in the future.

This fixes #3074 .

Upon commit it fails first as there are two instances which will be detected
(one is deliberate but will be changed too) .

t/03_syntax_check.t

@@ -0,0 +1,29 @@
+#!/usr/bin/env perl
+
+# Basics: are there semantic errors which are easy to spot?
+
+use strict;
+use Test::More;
+
+my $tests = 0;
+my $prg="testssl.sh";
+# Patterns used to trigger an error:
+
+#1
+printf "\n%s\n", "Testing for missing vars at left hand side in double square brackets ...";
+
+# I know this isn't nice but perl doesn't seem for this so great either
+my @matches = `grep -n '\\[\\[ [[:alpha:]]' $prg`;
+is(scalar(@matches), 0, "Checking bad '[[ LHS' patterns")
+    or diag(@matches);
+$tests++;
+i
+
+# more would go here
+
+printf "\n";
+done_testing($tests);
+
+
+#  vim:ts=5:sw=5:expandtab
+