workflow: test under valgrind

needed to workaround a broken interaction between valgrind and glibc

This fails due to #524

.github/valgrind.supp

@@ -0,0 +1,7 @@
+{
+   "workaround bug in valgrind"
+   Memcheck:Overlap
+   fun:__memcpy_chk
+   obj:/usr/lib/x86_64-linux-gnu/perl-base/auto/Cwd/Cwd.so
+   fun:Perl_pp_entersub
+}

.github/workflows/valgrind.yml

@@ -0,0 +1,37 @@
+name: valgrind
+
+on:
+  push:
+    branches:
+      - '**'
+    tags-ignore:
+      - '*'
+  pull_request:
+
+
+jobs:
+  imager:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - name: install dependencies
+        run: |
+            sudo apt-get update ||:
+            sudo apt-get -y install build-essential libtiff-dev libpng-dev libgif-dev libfreetype6-dev libjpeg-dev libtest-pod-coverage-perl libtest-pod-perl libpod-parser-perl libpod-spell-perl aspell libinline-c-perl libparse-recdescent-perl cpanminus valgrind
+            # prerequisites for Inline::CPP
+            sudo apt-get -y install libcapture-tiny-perl libmodule-build-perl
+            sudo cpanm Inline::CPP
+      - name: checkout
+        uses: actions/checkout@v2
+      - name: configure
+        run: |
+          perl Makefile.PL OPTIMIZE="-O0 -g"
+      - name: build
+        run: |
+            make -j2
+      - name: test
+        run: |
+            echo '::group::Imager'
+            IMAGER_AUTHOR_TESTING=1 prove -e 'valgrind --error-exitcode=1 --suppressions=.github/valgrind.supp -q perl -Mblib' -r t
+            echo '::endgroup::Imager'
+            for subdir in FT2 GIF ICO JPEG PNG SGI TIFF ; do ( cd $subdir ; echo "::group::$subdir" ; prove -e 'valgrind --error-exitcode=1 --suppressions=../.github/valgrind.supp -q perl -Mblib' t ; echo "::endgroup::$subdir" ) done