test: Use seperate test projects for bigframes integration tests (#16900)

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Fixes #<issue_number_goes_here> 🦕

Author: TrevorBergeron

.kokoro/continuous/continuous-bigframes.cfg

@@ -5,3 +5,8 @@ env_vars: {
     key: "NOX_SESSION"
     value: "e2e load system_prerelease notebook system_noextras"
 }
+
+env_vars: {
+  key: "BIGFRAMES_TEST_PROJECT"
+  value: "bigframes-load-testing"
+}

.kokoro/continuous/prerelease.cfg

@@ -5,3 +5,8 @@ env_vars: {
     key: "NOX_SESSION"
     value: "prerelease_deps"
 }
+
+env_vars: {
+  key: "BIGFRAMES_TEST_PROJECT"
+  value: "bigframes-load-testing"
+}

.kokoro/continuous/system.cfg

@@ -5,3 +5,8 @@ env_vars: {
     key: "NOX_SESSION"
     value: "system-3.12"
 }
+
+env_vars: {
+  key: "BIGFRAMES_TEST_PROJECT"
+  value: "bigframes-load-testing"
+}

.kokoro/presubmit/prerelease.cfg

@@ -5,3 +5,8 @@ env_vars: {
     key: "NOX_SESSION"
     value: "prerelease_deps"
 }
+
+env_vars: {
+  key: "BIGFRAMES_TEST_PROJECT"
+  value: "bigframes-testing"
+}

.kokoro/presubmit/presubmit-doctest-bigframes.cfg

@@ -15,4 +15,9 @@ env_vars: {
 env_vars: {
     key: "TRAMPOLINE_BUILD_FILE"
     value: "github/google-cloud-python/packages/bigframes/scripts/run_doctest.sh"
-}
+}
+
+env_vars: {
+  key: "BIGFRAMES_TEST_PROJECT"
+  value: "bigframes-testing"
+}

.kokoro/presubmit/system.cfg

@@ -5,3 +5,8 @@ env_vars: {
     key: "NOX_SESSION"
     value: "system-3.12"
 }
+
+env_vars: {
+  key: "BIGFRAMES_TEST_PROJECT"
+  value: "bigframes-testing"
+}