gdb/testsuite: Fix typos in infcall-nested-structs.c

gdb/testsuite: Fix typos in infcall-nested-structs.c

Some of the comparison functions in infcall-nested-structs.c contain
redundant comparisons like a.<some_field> == a.<some_field> instead of
a.<some_field> == b.<some_field>.  They were introduced with this commit:

  36eb4c5f9bbe6 - "infcall-nested-structs: Test up to five fields"

Fix the redundant comparisons.

gdb/testsuite/ChangeLog:

	* gdb.base/infcall-nested-structs.c (cmp_struct_02_01)
	(cmp_struct_02_02, cmp_struct_04_01, cmp_struct_04_02)
	(cmp_struct_05_01, cmp_struct_static_02_01)
	(cmp_struct_static_04_01, cmp_struct_static_06_01): Fix redundant
	comparisons.