[gdb/testsuite] Fix clean_restart <absolute filename> in gdb.dwarf2

[gdb/testsuite] Fix clean_restart <absolute filename> in gdb.dwarf2

Fix clean_restart <absolute filename> in the test-cases in gdb.dwarf2.

Tested on x86_64-linux using gcc-14.

Tested these test-cases on x86_64-linux with target board unix/-m32:
- gdb.dwarf2/valop.exp
- gdb.dwarf2/callframecfa.exp
- gdb.dwarf2/implptr.exp
- gdb.dwarf2/watch-notconst.exp
- gdb.dwarf2/pieces.exp
- gdb.dwarf2/pieces-optimized-out.exp

Tested these test-cases on x86_64-linux with target board fission-dwp:
- gdb.dwarf2/dwp-symlink.exp
- gdb.dwarf2/dwp-sepdebug.exp

Tested test-case gdb.dwarf2/gdb-index-tilde.exp on x86_64-linux by disabling a
too strict home directory check, see PR testsuite/33364.