Fix racy clashing of output files for gdb.base{1,2}/ parallel run.
* gdb.base/annota1.exp (thread_test): Import global $testfile.
Change the compilation target to match the testcase name.
* gdb.base/sect-cmd.exp: Change the compilation target to match the
testcase name.
* a2-run.exp: Use for setup prepare_for_testing. Change the
compilation target to match the testcase name.
* gdb.base/commands.exp: Likewise.
* gdb.base/finish.exp: Likewise.
* gdb.base/float.exp: Likewise.
* gdb.base/info-target.exp: Likewise.
* gdb.base/relational.exp: Likewise.
* gdb.base/term.exp: Likewise.
* gdb.base/until.exp: Likewise.
* gdb.base/volatile.exp: Likewise.
* gdb.base/whatis-exp.exp: Likewise.
