Add TESTING build flag to enable timer test_standalone on V61+

[zbelinsk]

Plumbs a TESTING define from the top-level makefile through kernel/Makefile into timer.ref.c and timer.v60opt.S, bypassing the V61+ core-timer-register read path so the test can drive the timer via device pointer registers.

Background:
On ARCHV > 60 (V61 and later) the hypervisor reads the hardware tick count from the per-core timer registers (TIMERHILO / H2K_get_timer_reg). These registers cannot be written from software, so kernel/time/timer/test_standalone

• which needs to set the tick count to drive the test - cannot run on those architectures. On ARCHV <= 60 the tick count is read from device-pointer MMIO registers (HW_COUNT_HI / HW_COUNT_LO) which the test can write, so the test already works there.

The TESTING flag forces V61+ builds to use the same device-pointer read path as V60 and earlier, making the test runnable. The flag has no effect on ARCHV <= 60 builds (they already use the device-pointer path unconditionally), so it is only meaningful for ARCHV > 60.

Changes:

• kernel/Makefile: forward TESTING to CFLAGS as -DTESTING
• makefile: pass TESTING=$(TESTING) to kernel sub-make for opt and ref
• timer.ref.c, timer.v60opt.S: gate the V61+ TIMERHILO path behind !TESTING
• scripts/testlist.v61, scripts/testlist.v81: re-enable kernel/time/timer/test_standalone
• .github/workflows/h2_reg.yml: pass TESTING=1 in the regression test step so test_standalone runs against the device-pointer read path in CI

Usage:
TESTING only takes effect at build time, and only for ARCHV > 60. The build and the test run are two separate steps - you must build with TESTING=1 first, and only then run the tests. Running the tests without rebuilding will exercise the normal (non-TESTING) code path.

1. Build the hypervisor with TESTING enabled (only meaningful for ARCHV > 60)

make TARGET=opt ARCHV=81 TESTING=1

(or TARGET=ref for the reference build)

2. Run the tests against the TESTING-built hypervisor

make TARGET=opt ARCHV=81 test

CI note: Because the testlists now include test_standalone, the GitHub regression workflow is also updated to pass TESTING=1 to make testall. This means CI exercises the device-pointer timer-read path on V61+ rather than the production TIMERHILO path. If preserving production-path coverage in CI is required, a follow-up can split this into two passes (with and without TESTING=1).

Without TESTING=1 the build behaves exactly as before, so existing CI flows that do not opt into TESTING are unaffected.