Tests if P-521 works on Safari

[jonasfj]

Comments in our code reference a bug claiming that P-521 doesn't work in safari:

webcrypto.dart/lib/src/webcrypto/webcrypto.dart

Lines 71 to 76 in e7fbe50

	///
	///
	/// P-521 is **not supported on Safari**, see [bug 216755 (bugs.webkit.org)][1].
	///
	/// [1]: https://bugs.webkit.org/show_bug.cgi?id=216755
	p521,

However, reading: https://bugs.webkit.org/show_bug.cgi?id=216755 this appears to have been fixed.

We should:

• Check if this is true,
• Enable tests covering P-521 on Safari / Mac
• Add additional tests if warranted.
• Remove the comment.