src.opts.mk: force the Dtrace tests off if ${MK_DTRACE} == no

Prior to this change Dtrace's tests could be enabled even if
`${MK_DTRACE}` == no. Allowing this doesn't make sense so remove the
tests if `${MK_DTRACE}` is also disabled.

MFC after:	2 weeks

share/mk/src.opts.mk

@@ -433,6 +433,7 @@ MK_MITKRB5:=	no
 # if it is off.
 .if ${MK_DTRACE} == "no"
 MK_CTF:=	no
+MK_DTRACE_TESTS:= no
 .endif
 
 .if ${MK_MAIL} == "no"