[gdb/testsuite] Fix clean_restart <absolute filename> in gdb.base, part 1

[gdb/testsuite] Fix clean_restart <absolute filename> in gdb.base, part 1

Run:
...
$ find gdb/testsuite/gdb.base/ -name *.exp* \
    | xargs sed -i 's/^\([ \t]*\)clean_restart $binfile$/\1clean_restart\n\1gdb_load $binfile/'
$ find gdb/testsuite/gdb.base/ -name *.exp* \
    | xargs sed -i 's/^\([ \t]*\)clean_restart ${binfile}$/\1clean_restart\n\1gdb_load $binfile/'
$ find gdb/testsuite/gdb.base/ -name *.exp* \
    | xargs sed -i 's/^\([ \t]*\)clean_restart $::binfile$/\1clean_restart\n\1gdb_load $::binfile/'
$ find gdb/testsuite/gdb.base/ -name *.exp* \
    | xargs sed -i 's/^\([ \t]*\)clean_restart ${::binfile}$/\1clean_restart\n\1gdb_load $::binfile/'
...

Revert the change in test-case gdb.base/foll-vfork.exp, which does
'set binfile $testfile'.

Tested on x86_64-linux.

The only modified test-case I was not able to test is
gdb.base/dtrace-probe.exp (filed PR testsuite/33379 about this).  The change
in the test-case is trivial though.