[Tests] Set LD_LIBRARY_PATH when running tests

swiftlang/swift#2609 introduced a change to
the default behavior of the Swift driver, which now uses the gold
linker unless specified otherwise. This caused a linker error in
swift-corelibs-xctest's test suite, which this commit fixes.

Author: modocache

Tests/Functional/lit.cfg

@@ -76,6 +76,7 @@ else:
         '-I', foundation_dir,
         '-I', core_foundation_dir,
     ])
+    config.environment['LD_LIBRARY_PATH'] = foundation_dir
 
 # Having prepared the swiftc command, we set the substitution.
 config.substitutions.append(('%{swiftc}', ' '.join(swift_exec)))