fate: add skip_clean option

This is useful if one wants to inspect build artifacts after running
fate.sh script.

Signed-off-by: Kacper Michajłow <kasper93@gmail.com>

doc/fate_config.sh.template

@@ -6,6 +6,7 @@ workdir=                                 # directory in which to do all the work
 #fate_recv="ssh -T fate@fate.ffmpeg.org" # command to submit report
 comment=                                 # optional description
 build_only=     # set to "yes" for a compile-only instance that skips tests
+skip_clean=     # set to "yes" to preserve build/install directories
 ignore_tests=
 
 # the following are optional and map to configure options

tests/fate.sh

@@ -95,7 +95,7 @@ fate()(
 )
 
 clean(){
-    rm -rf ${build} ${inst}
+    test "$skip_clean" = "yes" || rm -rf ${build} ${inst}
 }
 
 report(){