[gdb/testsuite] Fix use of verbose in gdb/jit-*.exp

[gdb/testsuite] Fix use of verbose in gdb/jit-*.exp

When running the gdb/jit-*.exp tests with runtest -v, I get:
...
ERROR: internal buffer is full.
UNRESOLVED: gdb.base/jit-elf-so.exp: one_jit_test-1: maintenance print objfiles
ERROR: internal buffer is full.
UNRESOLVED: gdb.base/jit-elf-so.exp: one_jit_test-2: maintenance print objfiles
ERROR: internal buffer is full.
UNRESOLVED: gdb.base/jit-elf.exp: one_jit_test-1: maintenance print objfiles
ERROR: internal buffer is full.
UNRESOLVED: gdb.base/jit-elf.exp: one_jit_test-2: maintenance print objfiles
ERROR: internal buffer is full.
UNRESOLVED: gdb.base/jit-elf.exp: attach: one_jit_test-2: maintenance print objfiles
ERROR: internal buffer is full.
UNRESOLVED: gdb.base/jit-elf.exp: PIE: one_jit_test-1: maintenance print objfiles
FAIL: gdb.base/jit-reader.exp: jit-reader-load
FAIL: gdb.base/jit-reader.exp: with jit-reader: before mangling: bt works
FAIL: gdb.base/jit-reader.exp: with jit-reader: after mangling: bt works
FAIL: gdb.base/jit-reader.exp: with jit-reader again: jit-reader-load
FAIL: gdb.base/jit-reader.exp: with jit-reader again: bt
...

This is the consequence of the use of global verbose in these tests, which is
used to change the actual test, rather than be more verbose about it.

Fix this by defining a global test_verbose in each test, and using that
instead.

Tested on x86_64-linux using runtest -v.

gdb/testsuite/ChangeLog:

2020-06-03  Tom de Vries  <tdevries@suse.de>

	PR testsuite/25609
	* gdb.base/jit-elf-so.exp: Don't modify testing behaviour based on
	value of global verbose.
	* gdb.base/jit-elf.exp: Same.
	* gdb.base/jit-reader.exp: Same.