Fix issue #1574 "fract in linalg/glm is broken" by fixing

trunc_f16/32/64 in "math.odin" (~ typos on expressions)
Fix classify_f16 Inf test (would fail for subnormal 0h0001)
  by changing multiplier 0.5 -> 0.25
Add some useful consts to "math.odin" (INF_F16 etc)
Add comment to "demo.odin" mentioning that -0.0 must be used
  to specify negative zero

tests/common/common.odin

@@ -0,0 +1,41 @@
+// Boilerplate for tests
+package common
+
+import "core:testing"
+import "core:fmt"
+import "core:os"
+
+TEST_count := 0
+TEST_fail  := 0
+
+when ODIN_TEST {
+    expect  :: testing.expect
+    log     :: testing.log
+} else {
+    expect  :: proc(t: ^testing.T, condition: bool, message: string, loc := #caller_location) {
+        TEST_count += 1
+        if !condition {
+            TEST_fail += 1
+			fmt.printf("[%v] FAIL %s\n", loc, message)
+            return
+        }
+        fmt.printf("[%v] PASS\n", loc)
+    }
+    log     :: proc(t: ^testing.T, v: any, loc := #caller_location) {
+        fmt.printf("[%v]", loc)
+        fmt.printf(" log: %v\n", v)
+    }
+}
+
+report :: proc(t: ^testing.T) {
+	if TEST_fail > 0 {
+		if TEST_fail > 1 {
+			fmt.printf("%v/%v tests successful, %v tests failed.\n", TEST_count - TEST_fail, TEST_count, TEST_fail)
+		} else {
+			fmt.printf("%v/%v tests successful, %v test failed.\n", TEST_count - TEST_fail, TEST_count, TEST_fail)
+		}
+		os.exit(1)
+	} else {
+		fmt.printf("%v/%v tests successful.\n", TEST_count, TEST_count)
+	}
+}