format numbers with a fixed Locale

Decimal/thousands separators differ per Locale. Using the default system Locale causes test failures when `,` is used as the decimal separator and `.` as the thousands separator. (which is common in Europe)`.

E.g.

```
Expected :83.3333 ops/us
Actual   :83,3333 ops/us
```

Author: adam-enko

runtime/jvmMain/src/kotlinx/benchmark/Utils.kt

@@ -1,10 +1,11 @@
 package kotlinx.benchmark
 
-import java.io.*
+import java.io.File
+import java.util.*
 
 internal actual fun Double.format(precision: Int, useGrouping: Boolean): String {
-    return if (useGrouping) "%,.0${precision}f".format(this)
-    else "%.0${precision}f".format(this)
+    return if (useGrouping) "%,.0${precision}f".format(Locale.ROOT, this)
+    else "%.0${precision}f".format(Locale.ROOT, this)
 }
 
 internal actual fun String.readFile(): String {
@@ -15,4 +16,4 @@ internal actual fun String.writeFile(text: String) {
     File(this).writeText(text)
 }
 
-internal actual inline fun measureNanoseconds(block: () -> Unit): Long = TODO("Not implemented for this platform")
+internal actual inline fun measureNanoseconds(block: () -> Unit): Long = TODO("Not implemented for this platform")