build: inline -enable-testing (NFC)

Inline `-enable-testing` into the two sites.  This makes it easier to
read.

Author: compnerd

CMakeLists.txt

@@ -79,9 +79,6 @@ set(swift_optimization_flags)
 if(NOT CMAKE_BUILD_TYPE MATCHES Debug)
   set(swift_optimization_flags -O)
 endif()
-if(ENABLE_TESTING)
-  set(swift_enable_testing -enable-testing)
-endif()
 if(CMAKE_SYSTEM_NAME STREQUAL Windows)
   set(swift_libc_flags -libc;${CMAKE_MSVC_RUNTIME_LIBRARY})
   if(CMAKE_SYSTEM_NAME STREQUAL Debug)
@@ -296,7 +293,7 @@ add_swift_library(Foundation
                     $<$<BOOL:FOUNDATION_ENABLE_LIBDISPATCH>:-DDEPLOYMENT_ENABLE_LIBDISPATCH>
                     -I;${ICU_INCLUDE_DIR}
                     ${libdispatch_cflags}
-                    ${swift_enable_testing}
+                    $<$<BOOL:ENABLE_TESTING>:-enable-testing>
                     ${swift_optimization_flags}
                     ${swift_libc_flags}
                   DEPENDS
@@ -401,7 +398,7 @@ add_swift_executable(plutil
                        -I;${CMAKE_CURRENT_BINARY_DIR}/swift
                        -I;${ICU_INCLUDE_DIR}
                        ${libdispatch_cflags}
-                       ${swift_enable_testing}
+                       $<$<BOOL:ENABLE_TESTING>:-enable-testing>
                        ${swift_optimization_flags}
                        ${swift_libc_flags}
                      DEPENDS