Issue Description:

This may be similar to SR-12599.

Download any developer snapshot from swift.org and try to run `swift test` in a package. The project will build, and then output the following error:

error: signalled(6): /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2020-04-13-a.xctoolchain/usr/libexec/swift/pm/swiftpm-xctest-helper /<redacted>/swift-format/.build/x86_64-apple-macosx/debug/swift-formatPackageTests.xctest /var/folders/lp/b8svs5jj46d_g3t6smn3cj3h006b5z/T/TemporaryFile.ZxlFMg output:
{{ dyld: Library not loaded: @rpath/XCTest.framework/Versions/A/XCTest}}
{{ Referenced from: /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2020-04-13-a.xctoolchain/usr/libexec/swift/pm/swiftpm-xctest-helper}}
{{ Reason: image not found }}

This seems to have broken sometime around the 5.2 cut, but I'm not sure exactly when.

If we run otool -l and compare the load commands of swiftpm-xctest-helper in a current toolchain and an older working toolchain, we see some differences:

Broken (2020-04-13):
Load command 32
cmd LC_RPATH
cmdsize 32
path /usr/lib/swift (offset 12)
Load command 33
cmd LC_RPATH
cmdsize 32
path @loader_path (offset 12)
Load command 34
cmd LC_RPATH
cmdsize 224
path /Users/buildnode/jenkins/workspace/oss-swift-package-osx/swift-nightly-install/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2020-04-13-a.xctoolchain/usr/lib/swift/macosx (offset 12)
Load command 35
cmd LC_RPATH
cmdsize 56
path @executable_path/../lib/swift/macosx (offset 12)

Working (2019-09-26):
Load command 33
cmd LC_RPATH
cmdsize 32
path /usr/lib/swift (offset 12)
Load command 34
cmd LC_RPATH
cmdsize 32
path @loader_path (offset 12)
Load command 35
cmd LC_RPATH
cmdsize 56
path @executable_path/../lib/swift/macosx (offset 12)
Load command 36
cmd LC_RPATH
cmdsize 56
path @executable_path/../lib/swift/pm/llbuild (offset 12)
...
Load command 39
cmd LC_RPATH
cmdsize 56
path @executable_path/../../../lib/swift/macosx (offset 12)

Specifically, it looks like the one that's missing in the newer toolchains is @executable_path/../../../lib/swift/macosx, and that is where XCTest.swiftmodule lives; presumably if it found that, it would be able to resolve XCTest.framework from that overlay.