gtest_premature_exit_test isn't being run correctly

**Describe the bug**

gtest_premature_exit_test expects to find an environment variable
"TEST_PREMATURE_EXIT_FILE" telling it where to find a file needed
to run the test properly. If this environment variable isn't set, the test
doesn't verify anything.

Running the test under `make test` doesn't set the environment variable.
I don't have Bazel set up, but grepping the tree doesn't find anything that
suggests it sets up the environment variable either.

**Steps to reproduce the bug**

In PrematureExitDeathTest.FileExistsDuringExecutionOfDeathTest,
put `assert(false);` or `EXPECT_TRUE(false);` before the
EXPECT_DEATH_IF_SUPPORTED call. It won't fire.

In PrematureExitTest.PrematureExitFileExistsDuringTestExecution,
put `assert(false);` or `EXPECT_TRUE(false);` before the
EXPECT__TRUE call; it won't fire.

The problem might be clearer if the early `return` in those test functions
used `GTEST_SKIP` instead; then the test wouldn't look like it worked.

If I manually set the environment variable, the test does work correctly.

**Does the bug persist in the most recent commit?**

yes

**What operating system and version are you using?**

Ubuntu 18.04

**What compiler and version are you using?**

gcc 7.5.0

**What build system are you using?**

cmake version 3.17.3

**Additional context**

Found by the Rotten Green Tests project (see issue #3811)