tests: fix comment declaring runtime in rv64 interruptedmemory test

The test attempts to trigger a regression for arount 30s. However, a
comment just before the computation of the target wall clock time falsly
declares the run time to be around 60s.

This was the case already when we introduced the test in

    5241645c47
    (tests: add test with interrupted memory accesses on rv64)

Signed-off-by: Julian Ganz <neither@nut.email>
Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

tests/tcg/riscv64/interruptedmemory.S

@@ -25,7 +25,7 @@ _start:
 	li	a0, 0x03	# 8N1, DLAB=0
 	sb	a0, 3(t1)
 
-	# Run test for around 60s
+	# Run test for around 30s
 	call	rtc_get
 	li	t0, 30
 	slli	t0, t0, 30 # Approx. 10e9 ns