Replace std.debug.assert with std.testing.expect in tests

[MorganMcmillan]

When a test fails as a result of std.debug.assert recieving false, it produces a rather large error message:

thread 11119 panic: reached unreachable code
zig/lib/std/debug.zig:412:14: 0x103eb9d in assert (test)
    if (!ok) unreachable; // assertion failure
             ^
zig-string/zig-string-tests.zig:24:21: 0x103e25f in test.Basic Usage (test)
    std.debug.assert(myString.cmp("🔥 Hello, World "));
                    ^
zig/lib/compiler/test_runner.zig:155:25: 0x1057a99 in mainTerminal (test)
        if (test_fn.func()) |_| {
                        ^
zig/lib/compiler/test_runner.zig:35:28: 0x104c7db in main (test)
        return mainTerminal();
                           ^
zig/lib/std/start.zig:505:22: 0x10479f9 in posixCallMainAndExit (test)
            root.main();
                     ^
zig/lib/std/start.zig:258:5: 0x1047561 in _start (test)
    asm volatile (switch (native_arch) {
    ^
???:?:?: 0x0 in ??? (???)
error: the following test command crashed:
zig-string/.zig-cache/o/eb36f7fdef19d5a1c7c2da449e45015e/test

But when try std.testing.expect is used instead, the error message looks like this:

1/7 zig-string-tests.test.Basic Usage...FAIL (TestUnexpectedResult)
zig/lib/std/testing.zig:542:14: 0x103e15f in expect (test)
    if (!ok) return error.TestUnexpectedResult;
             ^
Zig Projects/zig-string/zig-string-tests.zig:24:5: 0x103e3c4 in test.Basic Usage (test)
    try expect(myString.cmp("🔥 Hello, World "));
    ^
6 passed; 0 skipped; 1 failed.
error: the following test command failed with exit code 1:
zig-string/.zig-cache/o/eb36f7fdef19d5a1c7c2da449e45015e/test

Not only is the message shorter, but it's also clearer that a test failed, and where it failed.

std.debug.assert should not be used in testing, as it's meant for debugging programs. Always use std.testing.expect in tests to assert that a condition is true.