e2e tests: use /var/tmp, not $TMPDIR, as workdirs

TMPDIR is typically /tmp which is typically(*) a tmpfs.

This PR ignores $TMPDIR when $CI is defined, forcing all
e2e tests to set up one central working directory in /var/tmp
instead.

Also, lots of cleanup.

 (*) For many years, up to and still including the time of
     this PR, /tmp on Fedora CI VMs is actually NOT tmpfs,
     it is just / (root). This is nonstandard and undesirable.
     Efforts are underway to remove this special case.

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/utils/common_function_test.go

@@ -27,12 +27,6 @@ var _ = Describe("Common functions test", func() {
 		ProcessOneCgroupPath = defaultCgroupPath
 	})
 
-	It("Test CreateTempDirInTempDir", func() {
-		tmpDir, _ := CreateTempDirInTempDir()
-		_, err := os.Stat(tmpDir)
-		Expect(os.IsNotExist(err)).ShouldNot(BeTrue(), "Directory is not created as expect")
-	})
-
 	It("Test SystemExec", func() {
 		session := SystemExec(GoechoPath, []string{})
 		Expect(session.Command.Process).ShouldNot(BeNil(), "SystemExec cannot start a process")