meson.build: create __init__.py for sage.graphs.tests, too

The "tests" subdirectory is installed, so make it a package for
consistency. Add the generated __init__.py to .gitignore while we're
at it.

Author: orlitzky

.gitignore

@@ -462,6 +462,7 @@ src/sage/graphs/graph_decompositions/__init__.py
 src/sage/graphs/generators/__init__.py
 src/sage/graphs/__init__.py
 src/sage/graphs/base/__init__.py
+src/sage/graphs/tests/__init__.py
 src/sage/databases/__init__.py
 src/sage/stats/hmm/__init__.py
 src/sage/stats/__init__.py

meson.build

@@ -195,6 +195,7 @@ file_paths = [
   'src/sage/graphs/generators/__init__.py',
   'src/sage/graphs/__init__.py',
   'src/sage/graphs/base/__init__.py',
+  'src/sage/graphs/tests/__init__.py',
   'src/sage/databases/__init__.py',
   'src/sage/stats/hmm/__init__.py',
   'src/sage/stats/__init__.py',