Fix test file extensions in e2e tests (#408)

diff --git a/tfexec/internal/e2etest/testdata/test_command_failing/tests/passthrough.tftest b/tfexec/internal/e2etest/testdata/test_command_failing/tests/passthrough.tftest.hcl
similarity index 100%
rename from tfexec/internal/e2etest/testdata/test_command_failing/tests/passthrough.tftest
rename to tfexec/internal/e2etest/testdata/test_command_failing/tests/passthrough.tftest.hcl
diff --git a/tfexec/internal/e2etest/testdata/test_command_passing/tests/passthrough.tftest b/tfexec/internal/e2etest/testdata/test_command_passing/tests/passthrough.tftest.hcl
similarity index 100%
rename from tfexec/internal/e2etest/testdata/test_command_passing/tests/passthrough.tftest
rename to tfexec/internal/e2etest/testdata/test_command_passing/tests/passthrough.tftest.hcl