Bug 21717: Fix generation of real's values

With strict SQL modes we now have TestBuilder tests that fail randomly with:
  Out of range value for column 'tax_rate_on_receiving'
  Out of range value for column 'discount'

We are trying to insert a value that is too high for the type defined at DB level.

It happens for discount when a value > 100 is generated. The datatype is float(6,4).
From the MySQL Doc:
"""
MySQL permits a nonstandard syntax: FLOAT(M,D) or REAL(M,D) or DOUBLE PRECISION(M,D).
Here, (M,D) means than values can be stored with up to M digits in total, of which D digits may be after the decimal point.
"""

So float(6,4) does not allow 100.

To recreate easily the issue:
  use t::lib::TestBuilder;
  my $builder = t::lib::TestBuilder->new;
  $builder->build( { source => 'Aqorder', value => { discount => 100 } } )->{discount};

To make sure this patch fixes the issue you can use this script:
  use Modern::Perl;
  use t::lib::TestBuilder;
  my $builder = t::lib::TestBuilder->new;
  my ( $real, $i ) = ( 0, 0 );
  while ( $real < 100 ) {
      $real = $builder->_gen_real( { info => { size => [ 6, 4 ] } } );
      warn $i++;
  }
  warn "$i - $real";

Test plan:
0/ Does not apply this patch
1/ Run the snippet above
2/ Confirm that it fails quite quickly (constantly before 500 for me)
3/ Apply this patch
4/ Run again the script
5/ Confirm that it is stuck in an infinite loop
6/ CTRL-C :D

QA Note:
Other _gen_* subroutine may need a fix, but we will wait for them to fail
TODO:
1. Add JS check to the interface to prevent the use to enter a value > 100
2. Allow '100' as a valid value for discount (?)

Followed test plan, patch worked as described
Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>

t/lib/TestBuilder.pm

@@ -417,7 +417,7 @@ sub _gen_real {
     if( defined( $params->{info}->{size} ) ) {
         $max = 10 ** ($params->{info}->{size}->[0] - $params->{info}->{size}->[1]);
     }
-    return sprintf("%.2f", rand($max)+1);
+    return sprintf("%.2f", rand($max-0.1));
 }
 
 sub _gen_date {