[gdb/testsuite] Use $gdb_test_file_name in gdb.dwarf2

[gdb/testsuite] Use $gdb_test_file_name in gdb.dwarf2

In f.i. gdb.dwarf2/dw2-line-number-zero.exp we find:
...
    verbose "Skipping dw2-line-number-zero test."
...

Make the skip messages use the gdb_test_file_name variable, to reduce the
amount of changes that needs to be made when coping a test-case.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-20  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-error.exp: Use $gdb_test_file_name.
	* gdb.dwarf2/dw2-line-number-zero.exp: Same.
	* gdb.dwarf2/dw2-main-no-line-number.exp: Same.
	* gdb.dwarf2/dw2-ranges-base.exp: Same.
	* gdb.dwarf2/dw2-ranges.exp: Same.
	* gdb.dwarf2/dw2-vendor-extended-opcode.exp: Same.
	* gdb.dwarf2/var-access.exp: Same.