Enable tests for native_{expm1,tan,tanpi}

Bug: http://b/27512382

Change-Id: Ic19e1de9cd80e87453e28ad4f709ac7060f532c2
diff --git a/api/rs_math.spec b/api/rs_math.spec
index fa3899e..b4093e8 100644
--- a/api/rs_math.spec
+++ b/api/rs_math.spec
@@ -2309,7 +2309,7 @@
 t: f16
 ret: #2#1
 arg: #2#1 v
-test: none
+test: custom
 end:
 
 function: native_hypot
@@ -2679,8 +2679,8 @@
 w: 1, 2, 3, 4
 t: f16
 ret: #2#1
-arg: #2#1 v
-test: none
+arg: #2#1 v, range(-314,314)
+test: custom
 end:
 
 function: native_tanh
@@ -2728,8 +2728,8 @@
 w: 1, 2, 3, 4
 t: f16
 ret: #2#1
-arg: #2#1 v
-test: none
+arg: #2#1 v, range(-100,100)
+test: custom
 end:
 
 function: nextafter