Tests: make tests pass with SOURCE_DATE_EPOCH set

Fix tests to account for commit 243aed52 (cmTimestamp: Support
SOURCE_DATE_EPOCH to override current time, 2017-01-25).

In openSUSE we are running the test-suite as part of the build and that
failed in these two places when building everything with
`SOURCE_DATE_EPOCH` set.

Tests/CMakeTests/File-TIMESTAMP-NotBogus.cmake

@@ -1,6 +1,7 @@
 set(STAMP_FILENAME "${CMAKE_CURRENT_BINARY_DIR}/FileTimestamp-Stamp")
 set(STAMP_FORMAT "%Y-%m-%d")
 
+unset(ENV{SOURCE_DATE_EPOCH})
 string(TIMESTAMP timestamp1 "${STAMP_FORMAT}")
 
 file(WRITE "${STAMP_FILENAME}" "foo")

Tests/CTestTestStopTime/GetDate.cmake

@@ -49,6 +49,7 @@ macro(GET_DATE)
   # 01/12/2006  08:55:12
   # mm/dd/YYYY HH:MM:SS
   #
+  unset(ENV{SOURCE_DATE_EPOCH})
   string(TIMESTAMP "${GD_PREFIX}OV" "%m/%d/%Y %H:%M:%S")
 
   if(${GD_PREFIX}VERBOSE)