Skip tests that require `tar` if the binary is not available on the path

[bkhouri]

In trying to enable execution of Swift PM test in the Windows pseudo toolchain build (swiftlang/swift#80405), some SwiftPM tests fail with the following:

<EXPR>:0: error: UniversalArchiverTests.testSuccess : threw error "missingExecutableProgram(program: "tar.exe")"

This is a result of tar.exe not being installed on the Windows host.

The test should be guarded with try XCTRequiresBinary("tar") (or equivalent) which would:

• skip the test if the required binary is not accessible,
• execute the test if the required binary is executable