std.debug.assert: remove special case for test builds

Previously, std.debug.assert would `@panic` in test builds,
if the assertion failed. Now, it's always `unreachable`.

This makes release mode test builds more accurately test
the actual code that will be run.

However this requires tests to call `std.testing.expect`
rather than `std.debug.assert` to make sure output is correct.

Here is the explanation of when to use either one, copied from
the assert doc comments:

Inside a test block, it is best to use the `std.testing` module
rather than assert, because assert may not detect a test failure
in ReleaseFast and ReleaseSafe mode. Outside of a test block, assert
is the correct function to use.

closes #1304

std/buffer.zig

@@ -3,6 +3,7 @@ const debug = std.debug;
 const mem = std.mem;
 const Allocator = mem.Allocator;
 const assert = debug.assert;
+const testing = std.testing;
 const ArrayList = std.ArrayList;
 
 /// A buffer that allocates memory and maintains a null byte at the end.
@@ -141,19 +142,19 @@ test "simple Buffer" {
     const cstr = @import("cstr.zig");
 
     var buf = try Buffer.init(debug.global_allocator, "");
-    assert(buf.len() == 0);
+    testing.expect(buf.len() == 0);
     try buf.append("hello");
     try buf.append(" ");
     try buf.append("world");
-    assert(buf.eql("hello world"));
-    assert(mem.eql(u8, cstr.toSliceConst(buf.toSliceConst().ptr), buf.toSliceConst()));
+    testing.expect(buf.eql("hello world"));
+    testing.expect(mem.eql(u8, cstr.toSliceConst(buf.toSliceConst().ptr), buf.toSliceConst()));
 
     var buf2 = try Buffer.initFromBuffer(buf);
-    assert(buf.eql(buf2.toSliceConst()));
+    testing.expect(buf.eql(buf2.toSliceConst()));
 
-    assert(buf.startsWith("hell"));
-    assert(buf.endsWith("orld"));
+    testing.expect(buf.startsWith("hell"));
+    testing.expect(buf.endsWith("orld"));
 
     try buf2.resize(4);
-    assert(buf.startsWith(buf2.toSlice()));
+    testing.expect(buf.startsWith(buf2.toSlice()));
 }