[tests] avoid leaving generated files in CWD

Notably __fc_machdep.h in result dir, that could be picked up by other
tests in the same session...

tests/libc/runtime.c

 /* run.config*
    COMMENT: tests that the runtime can compile without errors (for PathCrawler, E-ACSL, ...)
-   CMD: @frama-c@ -print-machdep-header > __fc_machdep.h && gcc -I. -D__FC_MACHDEP_X86_64 @FRAMAC_SHARE@/libc/__fc_runtime.c -Wno-attributes -std=c99 -Wall -Wwrite-strings -o @DEV_NULL@
-   OPT:
+   CMD: FRAMAC='@frama-c@' %{dep:@PTEST_DIR@/runtime.sh}
+   OPT: %{dep:@FRAMAC_SHARE@/libc/__fc_runtime.c}
  */
 
 int main() {

tests/libc/runtime.sh

+#!/bin/sh
+set -e
+if test -z "$FRAMAC"; then echo "variable FRAMAC must be set"; exit 1; fi
+TMPDIR=$(mktemp -d fc_test_libc_XXXXXXXX)
+$FRAMAC -print-machdep-header > $TMPDIR/__fc_machdep.h
+gcc -I$TMPDIR -D__FC_MACHDEP_X86_64 $@ -Wno-attributes -std=c99 -Wall -Wwrite-strings -o /dev/null
+rm -fr $TMPDIR