tests/lavfi-regression: name temporary files after $testname in do_lavfi_pixfmts()

Fix a failure when running do_lavfi_pixfmts() for the
lavfi-tinterlace_merge and lavfi-tinterlace_pad tests concurrently.

Since they were using the same names for the temporary files, the first
ending test was removing them, and the second test was failing at
removing unexisting files.

tests/lavfi-regression.sh

@@ -86,10 +86,10 @@ do_lavfi_pixfmts(){
     filter_args=$2
 
     showfiltfmts="$target_exec $target_path/libavfilter/filtfmts-test"
-    scale_exclude_fmts=${outfile}${1}_scale_exclude_fmts
-    scale_in_fmts=${outfile}${1}_scale_in_fmts
-    scale_out_fmts=${outfile}${1}_scale_out_fmts
-    in_fmts=${outfile}${1}_in_fmts
+    scale_exclude_fmts=${outfile}${testname}_scale_exclude_fmts
+    scale_in_fmts=${outfile}${testname}_scale_in_fmts
+    scale_out_fmts=${outfile}${testname}_scale_out_fmts
+    in_fmts=${outfile}${testname}_in_fmts
 
     # exclude pixel formats which are not supported as input
     $showfiltfmts scale | awk -F '[ \r]' '/^INPUT/{ fmt=substr($3, 5); print fmt }' | sort >$scale_in_fmts