Tests/JBIG2: Add test cases for custom adaptive template pixels

I manually wrote a bunch of .ini files and ran this script to
produce the files:

    #!/bin/bash
    set -eu

    J=$HOME/Downloads/T-REC-T.88-201808-I\!\!SOFT-ZST-E/Software
    J=$J/JBIG2_SampleSoftware-A20180829/source/jbig2

    for t in '' template1- template2- template3-; do
      for p in '' '-tpgdon'; do
        i=${t}customat$p
        echo $i.ini
        cat $i.ini
        $J -i Tests/LibGfx/test-inputs/bmp/bitmap -f bmp -o bitmap-$i \
            -F jb2 -ini $i.ini
        cp bitmap-$i.jb2 Tests/LibGfx/test-inputs/jbig2/bitmap-$i.jbig2
      done
    done

The script's output (which shows the .ini file contents) was:

```
% ./make-custom-at.sh
customat.ini
-Gen -Seg 1
-Gen -Param -ATX1 -4
-Gen -Param -ATY1 -5
-Gen -Param -ATX2 6
-Gen -Param -ATY2 -7
-Gen -Param -ATX3 -8
-Gen -Param -ATY3 -9
-Gen -Param -ATX4 10
-Gen -Param -ATY4 -11
ENC Start ===>complete
customat-tpgdon.ini
-Gen -Seg 1
-Gen -Param -ATX1 -4
-Gen -Param -ATY1 -5
-Gen -Param -ATX2 6
-Gen -Param -ATY2 -7
-Gen -Param -ATX3 -8
-Gen -Param -ATY3 -9
-Gen -Param -ATX4 10
-Gen -Param -ATY4 -11
-Gen -Param -TpGDon 1
ENC Start ===>complete
template1-customat.ini
-Gen -Seg 1
-Gen -Param -Template 1
-Gen -Param -ATX1 17
-Gen -Param -ATY1 -2
ENC Start ===>complete
template1-customat-tpgdon.ini
-Gen -Seg 1
-Gen -Param -Template 1
-Gen -Param -ATX1 17
-Gen -Param -ATY1 -2
-Gen -Param -TpGDon 1
ENC Start ===>complete
template2-customat.ini
-Gen -Seg 1
-Gen -Param -Template 2
-Gen -Param -ATX1 17
-Gen -Param -ATY1 -2
ENC Start ===>complete
template2-customat-tpgdon.ini
-Gen -Seg 1
-Gen -Param -Template 2
-Gen -Param -ATX1 17
-Gen -Param -ATY1 -2
-Gen -Param -TpGDon 1
ENC Start ===>complete
template3-customat.ini
-Gen -Seg 1
-Gen -Param -Template 3
-Gen -Param -ATX1 17
-Gen -Param -ATY1 -2
ENC Start ===>complete
template3-customat-tpgdon.ini
-Gen -Seg 1
-Gen -Param -Template 3
-Gen -Param -ATX1 17
-Gen -Param -ATY1 -2
-Gen -Param -TpGDon 1
ENC Start ===>complete
```

Tests/LibGfx/TestImageDecoder.cpp

@@ -356,13 +356,21 @@ TEST_CASE(test_jbig2_decode)
 
     Array test_inputs = {
         TEST_INPUT("jbig2/bitmap.jbig2"sv),
+        TEST_INPUT("jbig2/bitmap-customat.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-tpgdon.jbig2"sv),
+        TEST_INPUT("jbig2/bitmap-customat-tpgdon.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-template1.jbig2"sv),
+        TEST_INPUT("jbig2/bitmap-template1-customat.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-template1-tpgdon.jbig2"sv),
+        TEST_INPUT("jbig2/bitmap-template1-customat-tpgdon.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-template2.jbig2"sv),
+        TEST_INPUT("jbig2/bitmap-template2-customat.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-template2-tpgdon.jbig2"sv),
+        TEST_INPUT("jbig2/bitmap-template2-customat-tpgdon.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-template3.jbig2"sv),
+        TEST_INPUT("jbig2/bitmap-template3-customat.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-template3-tpgdon.jbig2"sv),
+        TEST_INPUT("jbig2/bitmap-template3-customat-tpgdon.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-symbol.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-symbol-textrefine.jbig2"sv),
         TEST_INPUT("jbig2/bitmap-symbol-symbolrefine.jbig2"sv),