Minor: replace posix abs with kotlin.math

Do not use kotlin.test API in production sources

Author: ilya-g

core/nativeMain/src/Instant.kt

@@ -11,7 +11,6 @@ package kotlinx.datetime
 import kotlinx.cinterop.*
 import platform.posix.*
 import kotlin.math.*
-import kotlin.test.*
 import kotlin.time.*
 
 public actual enum class DayOfWeek {
@@ -212,7 +211,7 @@ public actual class Instant internal constructor(actual val epochSeconds: Long,
         actual fun now(): Instant = memScoped {
             val timespecBuf = alloc<timespec>()
             val error = clock_gettime(CLOCK_REALTIME, timespecBuf.ptr)
-            assertEquals(0, error)
+            assert(error == 0)
             // according to https://en.cppreference.com/w/c/chrono/timespec,
             // tv_nsec in [0; 10^9), so no need to call [ofEpochSecond].
             val seconds = timespecBuf.tv_sec.convert<Long>()

core/nativeMain/src/TimeZone.kt

@@ -8,6 +8,7 @@
 
 package kotlinx.datetime
 
+import kotlin.math.abs
 import kotlinx.cinterop.*
 import platform.posix.*
 import kotlin.native.concurrent.*

core/nativeMain/src/Util.kt

@@ -7,7 +7,7 @@
  */
 package kotlinx.datetime
 
-import platform.posix.*
+import kotlin.math.abs
 
 /**
  * Calculates [a] * [b] / [c]. Returns a pair of the quotient and the remainder.