qa_sources: disable execution speed sensitive checks

Disable certain sensitive checks for the processing rate when running
the instrumented build. This is done by introducing an Environment
variable which is used to conditionally skip them:
`DISABLE_SENSITIVE_CHECKS=1 qa_sources`

blocks/basic/test/qa_sources.cpp

@@ -55,7 +55,7 @@ const boost::ut::suite TagTests = [] {
         expect(eq(static_cast<std::uint32_t>(sink1.n_samples_produced), n_samples)) << fmt::format("sink1 did not consume enough input samples ({} vs. {})", sink1.n_samples_produced, n_samples);
         expect(eq(static_cast<std::uint32_t>(sink2.n_samples_produced), n_samples)) << fmt::format("sink2 did not consume enough input samples ({} vs. {})", sink2.n_samples_produced, n_samples);
 
-        if (std::getenv("DISABLE_SENSITIVE_TESTS") != nullptr) {
+        if (std::getenv("DISABLE_SENSITIVE_TESTS") == nullptr) {
             expect(approx(sink1.effective_sample_rate(), sample_rate, 500.f))
                     << fmt::format("sink1: effective sample rate {} vs {} +- {} does not match", sink1.effective_sample_rate(), sample_rate, 500.f);
             expect(approx(sink2.effective_sample_rate(), sample_rate, 500.f))