Remove hardcoded line number from the zeroize test

Instead, we insert a comment containing GDB_BREAK_HERE in the line we
want to break at, and let the gdb script search for it.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

tests/scripts/test_zeroize.gdb

@@ -31,19 +31,13 @@
 # the compiler potentially has a bug.
 #
 # Note: This test requires that the test program is compiled with -g3.
-#
-# WARNING: There does not seem to be a mechanism in GDB scripts to set a
-# breakpoint at the end of a function (probably because there are a lot of
-# complications as function can have multiple exit points, etc). Therefore, it
-# was necessary to hard-code the line number of the breakpoint in the zeroize.c
-# test app. The assumption is that zeroize.c is a simple test app that does not
-# change often (as opposed to the actual library code), so the breakpoint line
-# number does not need to be updated often.
 
 set confirm off
 
 file ./programs/test/zeroize
-break zeroize.c:100
+
+search GDB_BREAK_HERE
+break $_
 
 set args ./programs/test/zeroize.c
 run