Fix for Bug#56817 - mysqlhotcopy tests fail when DBI and DBD-mysql perl modules are not found

Third updated patch - this version also includes copyright notice in added Perl script.
This patch implements a check for such modules at runtime. If modules are not found or unable to load, the test is skipped with 
the following message:
            
            [ skipped ]  Test needs Perl modules DBI and DBD::mysql
            
Checks are done via a helper Perl script which looks for the module in a runtime environment that is as similar to that of the 
mysqlhotcopy script as possible (thus not intended for Windows environments at this time).
The helper script tells mysql-test about the result by writing information to a temporary file that is later read by mysql-test.
See comments in added files (have_dbi_dbd-mysql.inc and checkDBI_DBD-mysql.pl) for details.
The patch also removes the mysqlhotcopy tests from the list of disabled tests.

mysql-test/include/mysqlhotcopy.inc

@@ -4,12 +4,26 @@
 
 --source include/not_windows.inc
 --source include/not_embedded.inc
+--source include/have_dbi_dbd-mysql.inc
 
 if (!$MYSQLHOTCOPY)
 {
+  # Fail the test if the mysqlhotcopy script is missing.
+  # If the tool's location changes, mysql-test-run.pl must be updated to
+  # reflect this (look for "MYSQLHOTCOPY").
   die due to missing mysqlhotcopy tool;
 }
 
+# NOTE (johnemb, 2011-01-26):
+#       In this test mysqlhotcopy (a perl script) is executed as a standalone
+#       executable, i.e. not necessarily using the perl interpreter in PATH,
+#       because that is how the documentation demonstrates it.
+#
+#       We include have_dbi_dbd-mysql.inc above so that the test will
+#       be skipped if Perl modules required by the mysqlhotcopy tool are not
+#       found when the script is run this way.
+
+
 let $MYSQLD_DATADIR= `SELECT @@datadir`;
 --disable_warnings
 DROP DATABASE IF EXISTS hotcopy_test;