Make AvailabilityTest for packages optional

If missing we assume it is ReturnTrue which is what >= 100 of our packages use.

doc/ref/gappkg.xml

@@ -273,13 +273,6 @@ The following components of this record are <E>mandatory</E>.
   </Item>
   </List>
 </Item>
-<Mark><C>AvailabilityTest</C></Mark>
-<Item>
-  a function with no arguments that returns <K>true</K> if the package is
-  available, and <K>false</K> otherwise
-  (can be <Ref Func="ReturnTrue"/> if the package consists only of &GAP;
-  code).
-</Item>
 </List>
 
 The following components of the record are <E>optional</E>.
@@ -384,6 +377,13 @@ The following components of the record are <E>optional</E>.
   </Item>
   </List>
 </Item>
+<Mark><C>AvailabilityTest</C></Mark>
+<Item>
+  a function with no arguments that returns <K>true</K> if the package is
+  available, and <K>false</K> otherwise
+  (can be <Ref Func="ReturnTrue"/> if the package consists only of &GAP;
+  code; this is also the default value),
+</Item>
 <Mark><C>BannerString</C> or <C>BannerFunction</C></Mark>
 <Item>
   a string or a function, respectively,

lib/package.gi

@@ -865,7 +865,7 @@ InstallGlobalFunction( PackageAvailabilityInfo,
 
       # Test whether the availability test function fits.
       GAPInfo.PackageCurrent:= inforec;
-      test:= inforec.AvailabilityTest();
+      test:= (not IsBound(inforec.AvailabilityTest)) or (inforec.AvailabilityTest() = true);
       Unbind( GAPInfo.PackageCurrent );
       if test = true then
         LogPackageLoadingMessage( PACKAGE_DEBUG,
@@ -2524,7 +2524,7 @@ InstallGlobalFunction( ValidatePackageInfo, function( info )
                                   ForAll( l, IsString ) ) and
                          IsBound( r.filename ) and IsString( r.filename ) ),
         "a list of records with components `needed' and `filename'" );
-    TestMandat( record, "AvailabilityTest", IsFunction, "a function" );
+    TestOption( record, "AvailabilityTest", IsFunction, "a function" );
     TestOption( record, "BannerFunction", IsFunction, "a function" );
     TestOption( record, "BannerString", IsString, "a string" );
     TestOption( record, "TestFile", IsFilename,