Change `swt_` prefix to `swift_testing_`

[grynspan]

Change the prefix we use for (exported, in particular) C/C++ symbols from swt_ to swift_testing_. We picked swt_ originally when the package was experimental so as not to stomp on the Swift toolchain's namespace, but Swift Testing is an official part of the Swift project now and is distributed with the toolchain, so we should revisit.

swt_abiv0_getEntryPoint() will need to remain as-is for ABI stability.

The Testing Workgroup should review this proposal.