refactor(meson.build): check for autotest/ before calling subdir() (M…

…ODFLOW-USGS#1589)

* makes the autotest subdirectory of the proj root optional
* motivation: distribution doesn't contain autotest folder

meson.build

@@ -225,7 +225,10 @@ subdir('srcbmi')
 subdir('utils')
 
 # add autotest directory
-subdir('autotest')
+fs = import('fs')
+if fs.is_dir('autotest')
+  subdir('autotest')
+endif
 
 # meson tests to evaluate installation success
 testdir = meson.project_source_root() / '.mf6minsim'